If you can live with glassless ones, I have had good sucess in the past fabricating neg carriers DIY. I have made all sorts for odd historical formats that I print ocassionally from
I use black 4 ply matte board, a seres of sharp knife blades and a steel straight edge and combination square. Multiple plys of matte board gan be joined with spray on glue, such as Scotch 777, or joined using double sided Scotch tapes meant for their ATG line of tape guns. Hinges are made with clear iwde packing tape, sometimes the fibreglass reinforce kind if use is anticipated to be heavy.
Use an existing carrier, such as the 35mm that so many used enlargers come with, to template where the locating pins an side stops need to be replicated by notching one or more layers of matte board, and where the center of the optical path is to expand around when needing a larger format carrier.
For around the window where the neg sits, I black out the white core of the board layer(s) with a black Sanford Sharpie permanent marker.
Make sure that the condensor system can be set up or diffuser box covers adequately the larger format before making a good quality DIY neg holder.