M3 screws work for joining the two parts of the enclosure together (I used M3x16), but they are too wide to go through the pi's mounting holes... (and M2 are ways too small...).
Any other suggestion? (at this point, I'll try to glue inserts in the pi mounting holes so I can use smaller screws).
Edit: I'm using a pi3 rev B (I'm pretty sure it doesn't matter, but...)
PS: I tried both machine and metal M3's without any success