I just get an 8'x8' solar cover that would normally be used on a pool for customers who request it. It helps with the moisture bleeding into the cover and gives you a little extra mileage on your big foam cover. It also helps on evaporation a tad. My customers use them a lot in the summer as well as it will protect the acrylic and keep the evaporation rate lower, all while allowing the tub to not get too hot (trade off is it doesn't retain heat as well).
I'd be interested in hearing what you guys were referring to if you remember the product name, though.