How do I prevent shrinking and flexing?

I have made a custom enclosure for my car.  The box is made with the sub's mounted to it already.  I have removed my back seat and the box is bolted in its place.  Basicly the box is sitting in the trunk but the subs are no firing in the cabin and placed where the upright part of the back seat was.  I have made some plywood rings and am starting to make a skeleton cover for it.  The cover is to make the box/subs look like the back seat.  So there is going to be frame work aound the edges and connecting to the speaker rings.  I know I can pull the fleece over everything but from what I gather when you put the resin on the fleece it causes it to shrink and flex when drying.  Is there a way around this?  Like mixing the resin differently or anything?  Or maybe just apply resin around the edges, let it dry, then apply a few more inches, let dry.  Basically work my way from the outside in, ony an inch or two at a time.  this way the dried fleece wont be able to pull and stretch in such small areas?