You might consider using mount board, at least in the testing stage. Get a sheet of 4 ply mount board, cut it to the outside dimensions that will allow it to slide into the back of the enlarger. Next cut a hole for the negative that is slightly smaller than the outside dimensions of the negative. Get a piece of adhesive laminating film and stick it to both sides of the negative carrier and cut out the holes for the negative. What you can now do is use a high quality masking tape to tape the negative, by the edges ofr the corners, to the negative carrier, this allows you to pull the negative taut, so that the film doesn't buckle. By using the laminating film, the masking tape will come off of the carrier easier.

It is an inexpensive way to test different carrier configurations, that you can do without any machining. An exacto knife and a straight edge are about all of the tools needed. I have made numerous carriers of this type for odd size glass and film negatives, as well as adapters for 23C negative carriers to my 45MX enlarger.