Where are you located? Call Austin or Indiana. They will answer, respond and help. I've never had any luck with Fresno.
Better yet, go to the hardware store and buy a piece of 3/4 plywood, measure and cut in to two pieces (for a 2 piece bed of course). Any pop top owner can give you the sizes. Go to a foam/upholstery place and buy a piece of 4 inch standard foam to fit. Then have it upholstered somewhere.
Plywood (exactly what SMB uses): $40 and you'll have leftovers for another project or kindling
Foam (exactly what SMB uses): $100 at the very most, probably half that
Upholstery (the only tricky part of the equation): maybe $20 for material and free to make if you know someone who can sew. If you want to get really fancy add a zipper down one side. If you can't do it yourself I can't imagine it costing much to have two covers sewn up.
total: $160 I guess SMB is charging you the other $340 for the zipper or perhaps just because they are SMB. Enjoy!