Instead of trying to find a way to mount my last attempt at a PCB inside the echo dot, I added some mounting holes and sent it off to allpcb, and got the boards back in less than a week, not bad.

New versions:

Only change to the schematic is that the second input to PLL is the MLCK from the Dot (previously it was clkout from the WM8805). But it's not used; I figured it'd be nice to have the option though.

KiCad 3d render of PCB

Assembled PCB mounted in and connected to Echo Dot

Front view with 3.5mm optical connector in the side.

Side view of echo with new optical socket.

Problems:

• One of the tracks is a little too close to the edge of the board, and has been cut off - fixed with a bodge wire.
• That track is a power line, which was exposed on the edge and ended up touching the grounded dots heatsink when put back together. Not ideal. Resolved with a bit of kapton tape (not in the photo)
• It is a very tight fit (never coming out tight) - the mounting holes are slightly too small. On the plus side, the optical connector on the side does feel really solid...
• The whole pcb is slight to big. If it were a few mm's smaller in diameter, i think the echo dot would probably be able to close properly.

But it works - audio is 48kHz and sounds fine so long as something is plugged into the headphone socket (it sounds terrible without - I assume some EQ for the small internal speaker). So I'm going to call this "done".