I think you could probably use the spray glue found in the notions or quilting section of your fabric store.This should hold it together until you get the covers on it.You are covering it right??? Once the covers are on,they should stay together with no problem.
Do you have a picture of the bench??? That would help us to help you.
Kathi

I am not sure if you already have the foam, I did some dining room chairs, and a few other things with one of those foam mattress toppers, the kind at Walmart that look like egg cartons, the size can be anything from king to twin, we cut them in the pattern we wanted and put another cut piece on top, egg shapes facing and it turned out great, the chairs look great, it has been 3 years and they still have the same shape so I felt it was a good deal
rene

Go to an upholstery supply store. They will cut the foam to shape for you, and it will be a better quality foam at a lower price. If you need a spray adhesive, they will sell you that too.

I ordered a custom cut piece of foam from www.foamorder.com. I believe they also carry the spray adhesive.