You're not supposed to make your own, as the engineering has to be right or you die.
Having said that, I made my own; to overcome my inability to computer engineer and crash test the result, I just made it massively strong.
3 -1/2" square tube 1/4" thick wall, it pivots on 1-1/4" pins and locks with 2 x 1" diameter pins in the FEL mount.
Mounting plates are 3/8" x 4" mild steel. It weighs around 100lbs.
I wouldn't hang my life on 2x2 square stock, even if your machine is very light.