docs(shell): size the rear Ethernet, DC and power-button openings

docs/hardware.md

@@ -31,9 +31,15 @@ Same on every current `v1` shell (Ethernet, DC jack, power button). Old USB-C re
 | 2 or 4 each | M2.5 insert | Each of **3 Adafruit module bays** you populate (skip empty bays) |
 | 10 | M3×8 / M3×10 / M3×12 | Mount screen + lower front to the shell |
 | 4 | M3 insert | **Adjustable feet only** — omit if using solid feet |
+| 2 | M3 screw + nut, or M3 self-tapper | **Ethernet coupler only** — the flange screws into the two Ø4.0 mm rear holes, 27.0 mm apart. No insert: the holes are clearance, not insert pockets |
 
 Adafruit total if all three bays are used: **6 or 12** extra M2.5 inserts (2 or 4 per bay).
 
+The rear DC jack and power button clamp into their own Ø12.5 mm holes with the
+nuts they ship with — no screws or inserts. See
+[Rear I/O openings](parts/shell.md#rear-io-openings) for every rear hole
+dimension and what fits it.
+
 ### Screen bezel
 
 | Variant | M3 inserts |

docs/parts/shell.md

@@ -2,14 +2,105 @@
 
 The rear body. Pick one board-specific file. Every current `v1` shell has the same rear I/O:
 
-- **Ethernet** through the rear
-- **Barrel DC jack** for power
-- **Power button**
+- **Ethernet** — 16.0 × 14.0 mm opening with two Ø4.0 mm screw holes
+- **Barrel DC jack** — Ø12.5 mm
+- **Power button** — Ø12.5 mm
+
+Full dimensions and what fits them: [Rear I/O openings](#rear-io-openings).
 
 USB-C rear shells are **EOL** (USB-C spec compatibility) — see [`stls/shell/eol/`](../../stls/shell/eol/README.md).
 
 ![Pi and UPS mounted in the shell](../drawings/assy-01-boards.png)
 
+## Rear I/O openings
+
+Every current `v1` shell carries the same three openings on one sloped rear
+face, so these numbers apply to `Shell-x1202`, `Shell-x1206` and
+`Shell-x12-a1` alike.
+
+**The face is 3.0 mm thick.** That is the panel thickness a threaded connector
+has to clamp, so check that each part's panel-thickness range covers 3 mm
+before you buy it. Many panel connectors are cut for thin sheet metal and will
+not close on 3 mm of plastic.
+
+| Opening | Size | Centre (model X) |
+| --- | --- | --- |
+| **Ethernet** | **16.0 × 14.0 mm** rectangular cut-out (16.0 across the shell, 14.0 up the face), plus **two Ø4.0 mm screw holes 27.0 mm apart**, on the cut-out's horizontal centreline | 40.0 mm |
+| **DC power** | **Ø12.5 mm** round | 73.0 mm |
+| **Power button** | **Ø12.5 mm** round | 161.8 mm |
+
+All three centres sit at the same height on the face. Across the 200 mm-wide
+shell they run Ethernet, then DC jack, then power button: 33.0 mm from the
+Ethernet centre to the DC centre, and 88.8 mm from the DC centre to the button
+centre. Measured from the release STLs.
+
+### What fits the Ethernet opening
+
+A **panel-mount RJ45 feed-through coupler with a two-screw flange** — the kind
+that takes a patch lead on each side, not a punch-down keystone. Check three
+numbers on the datasheet before ordering:
+
+1. the part of the body that passes through the panel is **no larger than
+   16.0 × 14.0 mm**;
+2. the flange screw holes are **27.0 mm apart** and take **M3** screws — the
+   shell's Ø4.0 mm holes are clearance, so an M3 screw with a nut behind, or an
+   M3 self-tapper into the plastic, both work;
+3. it is rated for a **3 mm** panel.
+
+> [!NOTE]
+> **Do not buy from the reference model for this opening.**
+> [`reference-models/connectors/Ethernet_Panel_Coupler.step`](../../reference-models/connectors/Ethernet_Panel_Coupler.step)
+> is a **round coupler with a 21.7 mm thread**, contributed in
+> [#8](https://github.com/open-flight/openflight-enclosure/pull/8) as a
+> dimensional reference for *other* enclosure projects. This shell has no
+> round Ethernet bore at all — its opening is the 16.0 × 14.0 mm rectangle
+> above — so that part does not fit a `v1` shell. Match the three numbers
+> instead; no specific product is endorsed here.
+
+### What fits the DC opening
+
+A **panel-mount DC barrel jack** with:
+
+- a threaded barrel of **Ø12 mm or less** — the common 12 mm-thread panel jack
+  is the snug fit; an M11 jack also goes in, with 0.75 mm of slop per side that
+  its flange covers;
+- a **flange or bezel wider than Ø12.5 mm**, so it seats against the face;
+- a **panel thickness rating that reaches 3 mm**;
+- a **5.5 × 2.1 mm** socket, if you are feeding a Geekworm X1202 or X1206 —
+  that is the barrel size those boards use.
+
+Buy one that comes **pre-wired with a flying lead**. This build keeps soldering
+to a minimum, and a jack with solder tags adds a joint you would otherwise not
+have to make. You will still need an iron for the heat-set inserts, as
+[Required hardware](../hardware.md) says.
+
+Unlike the Ethernet one, the
+[reference DC jack](../../reference-models/connectors/DC_5.5x2.1_Panel_Jack.step)
+is representative: it is a round jack with an Ø11 mm threaded barrel behind an
+Ø15.5 mm flange, 18.5 mm long overall. That drops into the Ø12.5 mm hole with
+0.75 mm of slop per side, which the flange covers.
+
+> [!WARNING]
+> **Barrel-jack polarity — get this wrong and you can destroy the Pi, the UPS
+> and the radars at once.** A 5.5 × 2.1 mm plug carries no keying: a
+> centre-negative supply mates perfectly with a centre-positive jack and
+> reverses the rail. The Geekworm **X1202 and X1206 both expect centre pin
+> positive (+)** — confirmed by Geekworm in
+> [this exchange](https://github.com/open-flight/openflight/pull/273#issuecomment-5779406632);
+> their wiki pages do not state it either way, so do not go looking there.
+> When you wire the panel jack, the **centre/tip contact goes to +** and the
+> sleeve to −, and the supply you plug in must also be centre positive. Buzz
+> the jack out with a multimeter before the first power-up rather than
+> trusting wire colour. The openflight
+> [power guide](https://github.com/open-flight/openflight/blob/main/docs/get-started/power.md)
+> covers the wiring end to end.
+
+### What fits the button opening
+
+A **12 mm panel-mount momentary push button**. It must be **momentary**
+(spring-back): the X1202/X1206 read how long the button is held, so a latching
+or toggle switch will not work.
+
 ## Board-specific shells
 
 | Board | STL |