The two STL files are for the top and bottom of the body of the tricopter. I used 10mm square armatures and bolted the frame together with M3 hex screws and lock nuts. Friction keeps the arms in place which means if you have a crash they have some movement and tend not to snap.
I've used velcro on the top plate to attach my control board, and receiver. My lipo is mounted underneath with a scorpion strap and slots nicely between the bolts. The hole on the bottom is purely to reduce weight (if you're using the scad file then comment out the indicated line to eliminate the hole and print the top).