I've been using hot glue to 'pin' pieces together will shaping them.
When I tried it for seams, it gave me time to spread the bead out flat over the 1" foam but when I held the two pieces together it took so long to cool that it melted the foam and drew the glue away from the joint. Long seams with a zigzag worked up to about a foot before the starting point cooled too much to join.
I could play around with different temperature sticks, but drywall screws and bamboo skewers are doing the trick for me.
FWIW, I also tried softening the foam with a heat gun and pressing them together - failure.
Polystyrene cement (Testors model glue) - melts the foam before it cures. Anyone know of a solvent that will thin it but not eat the foam ? Mineral spirits (paint thinner) is fine on foam and (surprisingly !) acetone just seems to etch the surface whereas it positively EATS the beaded foam.