there's not that much to it, it just goes 10mm from the edge (half the width of the piece you're attaching) for a corner, or wherever you want the center of that other piece to be. It doesn't need to be super precisely located, all it does is allow you to get the tool through to tighten the bolt.
#Is there a jig stl for creating blind joints on 2020 extrusion
spiral glen
It would be pretty easy to cad up and make a drilling jig (just a L-shaped thing with a hole 10mm from the inside corner), though a 3d printed one is so soft the drill will easily chew it up on first use. A larger hole with a piece of tube pushed in works well though (something like brass or aluminium, hard enough to guide the drill but soft enough to not damage it). Or you just use the jig to guide a center punch or something and then locate the drill with that mark.
In fact several already exist:
https://www.printables.com/search/models?q=2020 drill jig&ctx=models
