You can make your own joining/mounting plates out of aluminum plate. I would at least grab the t nuts that slide into the extrusion slot from 8020 and use those in order to make life easier. Easiest thing to do would be to order a couple of joining plates from 8020 and use them as drilling templates for whatever you need to fabricate for mounting to the 8020. That is what i did for my machine and it seemed to work well so far
Another idea is to use steel square tubing to mount things like motors, especially if they need to be geared in some fashion, see left of this pic:
You can then order up 1/4-20 or 5/16-18 socket head screws from a place like enco and save $$ rather than paying through the nose for 25 screws.