I have never done it, but I have seen video of Platt Monfort out of Maine, using 2 - 1" X 4", and cardboard to make a strong, light, cheap strongback. You might try on his website. Although he is no longer with us, I heard his family still carries on. Aeroliteboats, or something like that. Sorry, but I recently had a computer crash and lost all my favorites and links.

Rod

: I have considered using a steel tube, but it will be too expensive.
: I'm not used to buy steel, so i don't where it's cheapest. But I
: found a squared tube at 100x100 mm (~4"x4"), 6 m(~20
: ft) long and 3 mm (~11 gauge???) thick (couldn't find it
: thinner). It costs 1830 Danish kroner (~351 $). Even if I could
: find it in half the thicknes and price, it would still be too
: expensive, compared to wood.

: Palle