I called around and there is a heating and air place that can order 30 PPI polyurethane foam that is 1" thick. I was wondering, could I buy two of these and 'sew' them snugly together with mono filament to make a 2" depth? Because if so, I can can get what I need for under $8. That would make things so much easier.
Any time you use something made for a different use in an aquarium you take some risks. If that foam was made for filtering air, for example, it could have a chemical treatment to kill molds, which would be very harmful in the water. If it was made for cushioning fragile items when packaging things, it could have other ingredients that might be harmful in the water, and it might have a limited life in water. Also consider the structural strength of the foam - air filters, packaging materials, etc. need very little structural strength compared to what a HMF needs. It might work fine, but if it doesn't it would be a big mess to clean up.