It sounds like a great idea to make the base adapted to fit on top of a 20x20 mm extruded aluminium beam. So we gave it a try and have a file available for test! :)
Please download "Base_on_2020_Al_beam_extrusion_v02.stl" and see if it works. It should work fine. The narrow surface could be a bit tricky to remain firm on the build plate but if you turn on extra adhesion such as raft or brim it should work fine.