To connect two aluminum extrusions at 90 degrees, you'd need to tap the center hole in one of them and drill a tool hole in the side of the other. But drilling an accurate hole in a little groove is not that easy without a jig. Here's one I made one evening.

This drilling jug uses standard metric nuts as drill guides (because it's far too easy to destroy pure plastic jig with a drill). Nuts will probably get mangled in the process too, but they're far more robust than plastic.

Obviously enough, the nut size you'll need is the same size you're drilling a hole for.
For 2020 (20mm) aluminum extrusion you will need a M5 nut (4.2mm drill bit).
For 1515 (15mm) you will need M4 (3.3mm drill bit).

Put a nut in the slot, slide the jig on the end of the aluminum extrusion and drill away. Very easy.

For yanks with metric deficiency: go buy some metric fasteners, for frogs' snakes!

Prints without any supports if you flip it so the round hole is on the floor.

License is BY CC-SA, meaning you can copy this in your projects, even in the ones you are going to make money on (or if you are a company), but you have to refer to the original design when you do so and provide the sources for your thingy (not the whole product, just this part), even if you modify it. Also, license you're giving it at should be compatible with BY CC-SA. You can google for if yours is compatible.

Sources in this case would be the STL files I've provided on this page and/or the OnShape public project by Toxuin (me), if you'll manage to find it.