Here is the (hopefully) final draft of the RFPD box. Alastair is currently drafting a design for the base, so any comments must come quickly.
For the front face, the idea is to mill a hole and cavity in a circular copper plate that fits snugly around the diode. The diode will then be held in place between the front of the box and this ring once it is screwed on. This will also serve as a thermal contact between the diode and the box, and we can make the contact better by applying some thermal compound between them. There are three 4-40 taps around the PD hole so that we can screw the plate on from the front.
As I mentioned before, I have spaced out the 1/4-20 taps in the bottom, 1-cm panel so that one of the holes is directly beneath the PD. This is so that we don't rotate the diode out of the beam path when we initially use a simple post to mount it to the table (while we wait for the solid bases).