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

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.