We are using 2" thick foam that I snagged from work. I am not entirely sure what it would be considered, however its thick enough and dense enough that I could hollow out a section and it should still hold its shape.
How high up the rear end should the patches go? we cut out the template and length wise seems good (about 1/2 to 2/3 down the inner thigh), however it doesn't come up very high in the back. Hopefully that made sense