For my frame corner fastening method I opted for single nuts (2 per corner) inset in a 3d printed corner bit and external bolts that tighten against the back wall of the aluminum channel.

Longer bolts attach the aluminum channel frame to the nose of the stock printrbot body. I put a blocking shim between the frame and body to provide a larger surface to spread forces upon.

I may put a block in the channel for rigidity and stability of the interface of the body and frame.

Other bits to develop include some sort of mount for the power supply and a spool holder.