For the wing’s main spar, I plan on using an aluminum trim channel. The two extruding sides will act as the spar caps to handle the bending moment while the connecting side will act as the shear web to handle the shear loads. Brackets will connect the spars of different subsections. Looks like most trim channel options use 6063 aluminum, but 6061 aluminum is about twice as strong with the same density so we should more strongly consider such options. Based on the material information, we want to develop a mass per length function for the spar.
Density = 2.7 g/cm^3 or 0.0030 slug/in^3
A = Height, B = Width, C = Thickness
Cross-Sectional Area = A * B - (B - C)(A - 2C)
Volume = Cross-Sectional Area * L = [A * B - (B - C)(A - 2C)] * L
Mass Per Inch = Density * Cross-Sectional Area * 1 = 0.0030 * [A * B - (B - C)(A - 2C)] slugs
If Square: Mass Per Inch = 0.0030 * [3 * S * T - 2 * T^2] slugs where S = side length and T = thickness
https://www.homedepot.com/p/Everbilt-3-4-in-Wide-Inside-x-4-ft-Aluminum-Trim-Channel-Heavy-Duty-6545/332733638
https://www.lowes.com/pd/Hillman-3-8-in-W-x-4-ft-L-Mill-Finished-Aluminum-Weldable-Trim-Channel/3053591
https://www.metalsdepot.com/aluminum-products/aluminum-channel-6061
https://www.metalsupermarkets.com/product/aluminum-channel-6061/ ← I like this one