I've been trying to find a futon mattress through freecycle but so far we don't actually have a bed for us except for the guestroom bed.

If you can't find a free futon, you can order through the JC Penney catalogue a three or four inch thick piece of foam in twin, double, and queen size for between 50 and 80 dollars (if I remember correctly). That can be used as a pretty good mattress on top of plywood. I recommend the extra firm 4 inch thick foam.