Last year we used pretty much all 8020 except for a few machined alumminum plates for various mountings. It works out really nice. However, the T-Nuts (the nice drop in ones) cost about $2 EACH. That was the most expensive thing on our robot I think, lol. This year we tried something different, their Anchor Fastners...they are really strong and form a perfect perpendicular angle (assuming you milled the edge properly). I have to say that being sponsored by Midwest Fluid Power, an 8020 distrubter has its advantages
One, it's completely FREE...and its a 4 mile drive to their warehouse to pick up pretty much anything we need (1010, 1020, etc) that they have in stock.