Have you considered using 1/2" foam with a layer of glass on each side.
3 pieces, each 3' wide, to form the spar.

Thanks, Ernest, I think this will probably end up being the way I go
to hold the costs down to a minimum while giving me the strength I
need. Since EPS foam is used in house construction for insulating
roofs, it is locally available and cheap.
If 1/2" foam leaves the spar too flexible, I can then easily go up
to 3/4 or 1 1/2" or more.
Now I just have to figger out how to get those layers of
fiberglass, (matt and cloth), on the inside of the hollow triangular
spar 8-]
best regards, Allen