This has been an interesting thread. One of the great advantages of SOF is lightness, but so are simplicity and construction speed. There is no denying the convenience of a simple plywood frame, and it is handy for compartments, but it is not as light as an equivalent solid wood frame - lightening holes notwithstanding.

Ignoring space-age materials and methods, the frame with the best strength-to-weight ratio is a steam-bent one, with a laminated frame equal or a close second.

The weight of a built-up frame is increased by the extra material at the joints. Some weight can be saved from a lap joint by tapering the members across the joint. There must be an optimum thickness-to-width ratio for the solid wood members, driven by the required glue area at the joints among other things: I suspect it is 3:1 for most woods.

If one can afford to use super-thin aircraft ply and build up a reinforced edge of solid wood, that would probably be the ultimate in strength-to-weight ratio and not too much of a hassle to build perhaps, but the ply cost is too rich for me.