Good Afternoon, Sherrie,
I'll second the comments above. Since you're using a Beseler, you have a real advantage over some other enlargers: the carriers are simple and flat so they can be made of almost anything which is sturdy enough to hold the negatives and is workable with easily available tools.
I've found that 1/8" tempered Masonite works very well. It's cheap, usually available in small two by two foot sheets at home centers, and is easily sawed and drilled by common wood-working tools. Be sure to get the tempered kind, because the standard kind leaves rough, "fuzzy" edges when cut and drilled.
All that said, it may be almost as economical to do as Jeanette suggests and pick up used carriers from E-Bay; there are usually various Beseler carriers listed, and, like other darkroom equipment, they often go for bargain prices.