Eric, I print 4x5s full-frame on an Omega D-2. I made my own carrier by buying a simple piece of glass, cut to a size that would fit where the negative carrier goes. Then I used black gaffers tape to mask off the areas where the negative does not sit. Instead of sandwiching with another piece on top, I just cut a top piece from black matteboard in the general form of the carrier, but with an enlarged hole whose edges do not come in contact with the negative. The purpose of this piece is to give the head something to rest on without touching the negative.
I use a cold light head so my negatives usually stay in place. But if one begins to curl, I tape it down with cellophane tape.
This works really well for me on the Omega. I've not used a Beseler but the idea works, assuming no structural reasons that prevent you from substituting glass for a carrier on the Beseler.
Sanders