steel really isn't that expensive either.
You can go to home depot, Pick up a couple maybe more 10 foot pipes.
If you don't know how to well or  don't have any more money for more steel, Build the frame out of wood.
2x4s are dirt cheap, support the shit out of it. drill holes through the pipie(works best if you drill through with a small bit first.
Screw it down tight, maybe put a little ply on the sides for extra support against wobble if you didnt screw it down the best.
Nothing like the solid feel of a nice steel rail. Maybe just my opinion but i think it's worth the extra buck.