Something I read a long time ago about a commercial FHM pond was they used waxed cardboard from butcher meat boxes for the FHM to spawn on. The waxed cardboard was free (they must have had a source), would float for the season, then sink and be gone for the winter.
Not saying it was good or bad as I have never tried it. Just something that I read.
My initial pallets sunk after about a year. My solution was to slide a couple strips of 2" foam insulation (I had surplus scraps) inside the pallet and held in place with a couple screws. These have lasted going on 5 years but I think I am going to need to pull them this spring. The nails are rusting out and they are starting to come apart. One already has. picture of one of my FHM spawning structures. note layer below water