By steel channel, do you mean the stuff that looks more or less like an
I beam that only has flanges on one side? That sounds very heavy to
me.

I would go down to you auto wrecking yard and see how cheap you can get
steel from a truck frame. The frame will be a lot stiffer for torsion
if you box the frame. I would not use any angle. See if you can find
some small channel that was used for industrial shelving. You could
also see if you can find some pieces for pallet racking. A couple of
10 foot beams might work real well for the long frame pieces. They
would already be boxed. According to my Grainger catalog , the heavy "
seismic " 9 foot beams are good for 7,000 lbs. I assume that is for a
pair, but it does not say.

Dan