Took in two bunk cushions (carried them under one arm). Shop owner said the fabric was fine, and just replaced the foam with the heavy dense stuff. Lugged the new cushions out one at a time. Still happy with them 12 years later.
He showed me how to test for good foam. Put the edge of the sheet against your chest, then press together with the heels of your hands. If they bottom out, try a different foam - at least for sleeping. Softer foam may be OK for sitting, but the dense is OK too (IMHO).
If you want a very dense foam, look at the closed cell stuff. I bit the bullet for this stuff (2") for the cockpit cushions about 25 years ago, and the foam is still fine. The sunbrella covers are getting a bit tired (too many slips with the clam dip) but are still servicable.
My recommendation for cushions is to forget the zipper - just baste the last seam (hidden) closed. When it comes time to open the zipper and remove the foam, the zippers are probably dead anyway and you'll still need the knife. Save the cost (time) of the zippers.