I'm not sure how you model an X-Ray out of playdough - but perhaps the kids can come up with a way? You might relax the rules with this playdough mat and ask them to model words ending with the letter x, too.
Alphabet Begin With The Letter X Playdough Mat
Why not make working with playdough one of your "letter of the week" activities - and print out this playdough mat, for the letter X, to help!
A zip begins with the letter z - but what else? Get the kids to brainstorm some "z" words and then model them out of playdough. I feel a playdough zoo coming on ...