3M 77 Spray Adheasive is some of the best for that kind of project. Some Autoparts stores carry it as well some home and hardware stores.
What style of foam are you using and how thick? If you have the space to make it 1.5-2" or more thick, you might try to layer the foam with 2 densities. The one layer boding to your seat would be a high density foam. and then the layer next to your helmet could be a softer density comfort foam that is .5-.75" thick. You can use the spray adheasive to bond the foam layers together as well.