The enclosure has been designed with Google Sketchup for a material thickness of 3mm. The standard enclosure is cut out of black acryl plastic (one side frosted) by a laser cutting service.
If you use the Lepton2.5 without the external spot sensor, utilize the "radiometric" variants of the main enclosure and the bottom part (has no hole for the spot sensor).
You can order the 2D laser cut side plates from Ponoko USA: http://www.ponoko.com/design-your-own/products/diy-thermocam-v2-enclosure-13930.
Formulor.de also offers a laser cutting service for that material in Germany. Seeedstudio.com has a low-cost laser cutting service in China, but their material thickness varies, so be careful.
You can also get the enclosure from me against a small donation, have a look at the part list for the link.
Or use a 3D printer to make your custom version of the enclosure, together with the 3D design files.