I've engineered a construction technique to make a weight efficient U/L spar. I want to bounce it off y'all for any problems or suggestions.
For the spar caps I start with a very long piece of thin wall aluminum tubing, about 5/8" OD, and cut about a 1/8" wide slot down its full length.
I lay it down flat with the slot facing up, tape off its ends, and fill it about 1/4 full with high strength epoxy glue.
Then I drop full length carbon rods through the slot, until the tubing is 2/3 or 3/4 of the way full of carbon rods and glue.
Then I push the edge of 1/8" carbon and/or plywood sheer web sheeting into the slot, until it touches the far side of the tubing. The sheer web can be made in segments. The sheer web sheeting would have to be supported with a long jig until the glue dries.
Then I make the second spar cap the same way, and push the sheer web sheeting into it.
Finally the aluminum tubing could be peeled off or not. The result is a sheer web with a bundle of carbon rod glued to its top and bottom. It should be quick and easy to make.
Please forgive the crude drawing.