An inside L bracket suitable for OpenBeam aluminium extrusions.
Print and go. I printed mine on a Replicator at reasonably high heat in ABS (230 non-accel) at 100% infill and got a surprisingly high strength part. The holes MIGHT be a little small depending on your print settings, so be aware and be prepared to drive the screw through with a little pressure.
OpenBeam can be purchased here: