The link @io2work gave is good. We also have a .WRL file of the board, but note that the component models are not 100% accurate and that it should only be trusted to maybe 0.5mm. The PCB dimensions and hole positions, however, are correct.
Also a general note on metal enclosures for electronics: they conduct electricity and block electromagnetic waves.
This means that you'll need to be careful to allow for enough clearance around any exposed metal on Tessel (to avoid shorting out any part of it) and find a way to keep the WiFi antenna (the white strip in the top right corner of the board) from being covered by the case if you want any hope of connecting to the internet. For optimal RF performance, there shouldn't be any metal surrounding the antenna on any side, which poses an interesting challenge for a case.