[GME]

Buy some 3x3 inch or another size you think would work, 3/8 inch thick aluminum angle. Brace from the inside so you don't hit your legs on it. Drill the legs with 5/16 or 3/8 bolt or make plates.
I thought they had twist in or other special bolts for those slots you could use?

There are a lot of options surrounding 80/20, but the profile size is what it is. Nothing I can do about the 3 x 3 size.

The 3 x 3 is attached with fasteners that fit in a special 13/16" milled hole at the ends. There are twist-in bolts, but they won't work in this application. With the special milled fastners, I'd have to use roll-in t-nuts, which are just okay. The milled fasteners are designed to use a 3" long t-nut with maximum bearing surface. Roll-in t-nuts bear enough to hold, but don't have all that much bearing surface. Makes sense. Otherwise, you couldn't roll them into the slot. Adding the additional 80/20 lengths can be made to work, but the installation won't be as stiff as it would be if installed with the proper hardware. Just the way it is.

The twist-in bolts should work for mounting the legs to the floor. So would roll-in t-nuts. I keep both around, so it's just a matter of selecting the one I like best. The twist-in are much more limited, because there aren't many bolt sizes. I'll have to check, but I don't think they are long enough to mount 3/8" material. With the roll-in t-nuts, you can choose the bolt length you need. I think they may have twist-in t-nuts, but I'm not sure. If they do, twist-in nuts may work all around.

Thank you for your suggestions, Bill. Much appreciated.

Gary