Right you are, Stacy! If you trim that "surplus" rubber out before attaching the frame to whatever mounting system use use, that piece can be cut using fancy edging scissors or a template cutting system (Coluzzle or Fiskars) and you might even be able to be at least partially cut it with a die cut machine. You may have to finish cutting the shape you just made with scissors if the die cut or swivel blade doesn't cut all the way thru' the rubber, but you'll have a line to cut on to make it easy. Then take your new stamp and mount it like you do your other stamps.
That extra piece of rubber can also be used to make reversed images.
One can also make "custom" simple shape & shadow stamps from fun foam sheets.