goodwill is a decent place to get lucky as are garage sales. i found all my new foam at these two.not reliable ,but i was in the right place at the right time. for material, we used a set of drapes from good will. new condition, lined and cheap---29 bucks with material left over.thre were lots of color choices as people do a lot of decor changes.we didn't sew, just fold over backboard and staple.they are holding up nicely after 2 years of camping.if the right place and time arn't available,foam is expensive and maybe cheap price on materials would offset costs.
wes