This is a completely customizable T-bracket for T-slots. You can set the number of holes in each direction, the thickness, the width, etc. It is designed with the idea that you basically just measure three pieces of hardware (your aluminum profile, your inserts, and your screws) and enter that information. Everything else can be tweaked to your liking, but is all based on those fundamental dimensions. I've also included the STL files for three brackets I typically use, sized for 2020 extrusion with varying numbers of 5mm screw holes in each direction.
If you need a corner ("L"-shaped) bracket you can find that here: http://www.thingiverse.com/thing:1876482