Building L sofa Need Help & Suggestions (Bob Villa)

[LI Pets]

I building a L shaped couch sofa, I laid out the first set of frames.

I used 3/4" wood cleats on the floor to uprights so far, I was thinking of using light guage L alum on walls to uprights.

The left side are 33" apart and the right are 24"" apart, I think I need another on the left side.

The (bottom) fronts are going to be fixed with either 1/4" or 3/4" cherry ply.
I would like to save the weight and $$ on the 3/4".

The seats are going to have hatches about 18" Wide by 10" front to back, but be fixed solid otherwise. I was going to use 3/8" ply but it will need support?

The seat backs also 3/8"

How would you suggest I frame out the rest of the supports or any other changes.

Thanks in advance.