I made my sprung strongback from roughsawn 6x1 boxing grade radiata pine. i.e. the cheapest solid timber I could find. Easy enough to clean it up, and it came in 6m lengths so no joins were needed.

Cost me around $20 including screws. Much cheaper than plywood at ~$80 per sheet.

I haul it up into the ceiling of my shop loaded with 75% built kayak and all the remaining strips with a 2:1 pulley system. I can easily carry the bare strongback around in one hand.

Plenty stiff, (no twist or curve like you might see in a 2x4) and after over 2 years of being suspended from the strings it is still perfectly straight and true.

I left the uprights on the "top" of my strongback after I disconnected the forms after stripping the hull, flipped the strongback and screwed some padded wedges onto the "bottom" to cradle the hull while I build the deck. I figure that way the next time I come to build a boat, I just flip it back over and it's ready to have the forms attached.

If space is an issue, you could make it narrower. I built mine the same width as my kayak, but made it slightly longer. I may be able to dig up my spreadsheet with all the calculations if that helps - or you could take the easy road, and just screw the cross members onto the top of the sides.