Shown here are plans for my office/bedroom build (bed is a Japanese futon and pillows that fold and fit into the compartment of the 3 benches). As you can imagine, if I need to sleep, I can set the futon on the floor or, put these benches inline, and set them on cushions (latches on the bottom to prevent them from coming apart). These benches will be set on 3" casters (not shown).
The problem I'm trying to solve is how to design in a sturdy deployable backrest. What's shown is a crude T-bolt and knobs implementation but I don't think it'll hold up when a person is leaning against it. It might work if I use two knobs per slot but the short height of the backrest won't leave room for a second knob.
The benchseats carcass and backrest will be made from 3/4" hardwood plywood (Home Depot/Lowes). Where stiffness is needed, I prefer to double-up on the plywood.
Willing to use metal (1/4"-thick will do I think) in the design if it solves the problem. I'm thinking a metal L-bracket that latches onto the underside and sides of the bench (that's how my computer chair solved the problem).
I'm thinking to widen the slot to fit one of these tubes. The base of the tube, fixed to the base of the seat, would be 12" tall. The inner telescopic arm would be attached to the lifted backrest. Assuming the base tube and the telescopic arm overlap 4", the backrest should come up 8". If I attach the base tube inside of the seat, I can hide the hardware and have it extend lower to the ground by 2". The cushion piece can fit over the 3/4" plywood (bridle joint) and increase the height by another 4".
Update 2 Telescopic tube could work but is more complex to integrate into the design. I think a ratcheting bracket might work better. Something like Amz: 180 Degree 5 Gear Folding Chair Lift Bracket Sofa Hinge. Mount that upside-down so that when the bracket is fully opened (the bracket arms are 180 deg to each other), the backrest is "deployed". When it's closed (bracket arms are pointing in the same direction, downwards towards the floor), the backrest is "stowed". Amz: This looks pretty promising
Update 3 Another idea (I'm on a roll today): Slotted backrest! I think I saw this on youtube somewhere ... Modern Builds/Plywood Sofa?