There are two ways I have glued foam to spandex (successfully):
(1) Sewing it on. You have to use contact cement to glue a separate fabric to the back of the foam (I've used
tempo) then sew the tempo to the spandex. I suggest using a
curved needle to do this, when I discovered those it was a life saver- no more trying to get a straight needle to turn!! This is time consuming but I personally think its one of the best ways to keep the flexibility of the spandex and still have the foam securely attached.
(2) If you have a mannequin to stretch your spandex over you could use contact cement straight on the spandex and the foam... but find something to put between the mannequin and the spandex (maybe wax paper? just something the contact cement won't sick to) or else you will have a mannequin permeable attached to your spandex once the contact cement dries...
I've also tried hot gluing the foam to spandex but I really don't suggest it. The glue doesn't have a strong bond with the foam, and it has pulled off from almost every point on my costume where I used hot glue. I repaired it with contact cement and I hasn't budged since!
Also some sort of silicone glue would be an interesting thing to test.... I've never tried it but it's an exciting idea!
