you can do it your self, its real simple. all you need is some hard foam, glue (im not shore what type, but pva should work or even hot melt glue), material, vinal is good but wetsuit material is better and a good strong staple gun.
all you do is pull the old cover off, cut the new peice of foam to the shape you want then glue that to the top of the old one, then stretch the new material over the lot and go nuts with the staple gun.