Building a composite spar?

My dirigible design requires a few long spars curved into C-
shapes. They would be triangular cross-section with their three
faces being about 3 feet wide and 100 feet long. They need to be
light, strong and stiff. [what's new?]
If anyone's has experience building large spars or frame members, I
would like some advice as to how to make them cheap and easy.
My present thoughts favor a thin fiberglass skin around a bundle of
polypropylene tubes, which could get expensive.
I think that EPS blocks carved into the triangular shape and then
covered in glass would require the extra labor of building a glass
internal spar, yuch.
This design has to be a cheap and dirty way for the working guy to
get in the sky.
thanks from Allen