This brace is designed to fit mil-spec carbine length AR-15 tubes.

No support should be needed to print.

The locking pin hole can be drilled with a letter "N" ( 7.7mm ) drill if you have one.  For assembly two 3/32" ( 2.5mm ) pins are used and a 1/4" ( 6mm ) diameter spring sits in the slot under the lever to actuate the locking pin.

STL and STP files provided