These are perspective-corrected images of the Raspberry Pi beta boards which can be used to take measurements from. I estimate that they are accurate to within 1mm.
Note that most printer/program combinations will not print them the correct size, even if you tell them how big you want them. You will need to do some test prints and scale them by trial and error. You need them to print exactly 85mm by 56mm.
Note that these pictures cut off the connectors at the board edges. To determine where to place cutouts for the connectors, use Gert's mechanical drawings. You should use these images solely for determining where to place support structures.
I intend to use polystyrene sheet to make the box, and this is the support structure I intend to use:
It uses two structures: one is a shelf 20mm x 5mm on the corner of the board, the other is a bar shaped like this and placed 25mm in from the other end of the board. That's a measurement to the bar, not its centre-line.
Note that the tops of those pillars should be padded, since there are PCB traces under it, and the padding must be taken into account when determining the height of the pillars.
In order to hold the board in place there should be another pillar bearing down on the board from above. A suitable position is above the support bar and also 25mm in from the top edge (with the video and audio connectors.) Almost directly above the Raspberry Pi logo. When the case is closed the top pillar and the bottom pillars should have a vertical gap of around 1.6mm between them (plus solder-resist and silk-screening). Remember that this should allow for compressed padding on all pillars.
In the end, the real test is if the board is held securely without flexing.
