I'm planning on doing the same thing for my SCAR H. I've seen other people have done it and as others have said, they printed them in two halves so the ridged run inline with the path of the BBs. I'm also going to use existing mags as donors to get the springs, catch and follower from, as well as copying the internal dimensions. I think the best looking and strongest way of building it would be to have your two halves as internal pieces and then have a seperate sleeve that they slot into. That way you're separating your functional parts from your aesthetic parts.