I have completed my build today, it is based around
an Hammond 1590Y (9x9x3.8cm) enclosure
a 4x4cm 12706 TEC Module and
an old CPU heatsink, choosen for its massive Al base plate.
4 M5x20 Nylon screws clamp everything together. Insulation is 10mm Aerogel, used about 20x30cm of the stuff. Gloves and a vacuum on hairline trigger recommended. Easy to cut with a sharp kinfe, can be drilled. Tools without rubber overmolding preferred, the dust loves to stick to it. Dust wipes away with some wet paper towels.
At 12.5V i reached -12°C inside (after about 15 minutes). Outside around the lid felt cool to the touch, so if i were to do it again, i would double up the insulation on top just to have a longer sealing area. The double insulation on the side where the wires come out, with the wires making an u-turn between the layers seems to work pretty good to stop thermal flow (wires don't feel cold).
Imgur Album with build Pictures:
https://imgur.com/a/gfpLU76Now to build some control electronics...