This is a tube clamp that i designed based on a metal version i saw online. It joins two or three 1" tubes at a 90º angle in two directions. It is made for a bolt and nut to clamp the pieces together. 

Print 1 part 1 and two part 2.

For maximum strength print with the orientation that the stl has, it will need support, but that way the layer lines give it the best strength.

I printed these in ASA, but PETG, ABS, and PA(nylon) should all give it enough strength.