A Bimini makes a pretty good sail if you lower one-half to the bow tubes, then support the aft portion with an oar and lines running back. Go for a big crescent-shaped pocket. This of course only works straight downwind, or nearly so. I've sailed away and around the bend from my party several times using this and other improvised sails like a tarp and an oar. But then you have to wait up. I'm a professional sailor, and I've often thought about jury rigs. Physics work against you. Mast made of oar(s) first. It gets forced forward and down. You practically need some sort of base. Lash all you want, strap, brace, it's hard. Get that dialed in, then you have a mast that is too small to move 1500# You need more like 20'. A kite would be good, but hard to rig, and expensive to crash into the brush. Windsurfer masts are light, but hardly taller than an oar. I gave up on sailing rafts. Here in Utah, you get a six-horse outboard, and it lasts as long as you take care of it. It's heavy, but they give you versatility.