George,

I made a plywood beam about 8"x8"x8' with internal reinforcement - with edge stringers to attach the sides together and 8"x7.5" interior webs to keep it square.
It takes 2 (+ partial lengths in your case). Then I used exterior splice plates on all 4 sides. The splices were just screwed in place with short deck screws. The upper splice plate might have to be put inside to get a smooth upper surface.
1/2" ply.
Very stiff. Mounted on some scrap 2'x12" boards for legs.

Works great. Mine had a slight twist, doesn't matter I just level the frames when I install them.

Have fun.