This works as a way to mount tube on the side of other tube. (handy to make frames out of straight aluminum tube without the skills to weld) 

Easiest way to print this is to face it up, no supports required then. 

These are insanely strong, built to use M4 Bolts (5mm diameter) tapped into the Aluminum, I was unable to crack one of these parts by hand and you can make them thicker than I have as it's a step file, modify as you want.

Comes with a piece to figure out where to tap your holes, put it on the end of your tube and use a hole tap where the holes in the guide part meet the metal. You should get them alligned fairly well. I tapped both sides of my metal after drilling however a bolt 60mm long and with a nut on the other end would also work.

Enjoy!