My son wanted something a little different than what he was seeing in the aftermarket arena and we're adding picatinny rails to his blasters. The paint job will probably be OD green and black.
The idea was to keep the crosshair its own model, so we could easily design new ones. The reality is that everything fits so tight that, once it's in, it's not coming out. Same goes for the rear part of the tube.
EDIT: Made the recesses on the side of the scope shallower, so it isn't paper-thin towards the back. Also added some chamfers to the clamping sections.
I print in ABS and always use supports when I have screw holes and recesses for nuts or area like the picatinny clamp that I want to be a perfect fit. I used rafts just because I always worry about the supports coming loose, especially on a longer print. You could probably get away without them.
I didn't need much, at all. The crosshair cooled just slightly wider than spec and some of the picatinny cutouts were a hair tight.
The crosshair slides into the rear opening of the housing and there are some notches for alignment. Once I had the crosshair just inside, I pushed it in with the eyepiece (there are notches for that connection, too). I was able to twist the eyepiece a little as I worked it together. Once it's in place, the crosshair should be perfectly aligned with the housing.
The picatinny clamp is sized for (1) M3-0.50x20mm screw and nut.