We purchased some custom cut foam (I think it was 4 inch) for a prior Airstream (1967 24 ft. Tradewind) restoration effort. See attached thread, posting #23 I think, where it shows the foam for the twins and for the bolster pillows (is that what they're called????).
I think it cost us somewhere around $500 for the 4 pieces of foam. I know that you're a long ways from Austin ... but if your travel plans ever bring you this direction then you could send them your dimensions and pick up the finished product on your way through.
Following is where we purchased the foam:
10201 Metropolitan Drive, Austin, TX 78758-4944