Stamp adhesive
If the stamps are new and PSA (pressure sensitive adhesive, or peel and stick), I find their own adhesive sufficient. However, you need to seal with CA before casting, and put an extra small drop of CA at any seams on the ends of the blank. If not, the PR will attack the adhesive, cause the tamps to lift off the tube, and the blank will be ruined. DANHIKT!
If the stamps no longer have adhesive, or if they are the older mucilage type (lick and stick), I use Elmers extra strength craft glue stick. This works well for me, and gives a short time to reposition the stamps before it locks them in place. Finger pressure for maybe 30 seconds is sufficient. You should seal these also with CA before casting.
I avoid Mod Podge like the plague. I have had too many blanks turn milky after casting, no matter how well or how long I thought I had dried them.
Jim