Experiment with laminated sections of matt board off cuts first. You might find the DIY matt board version, with a bit of tape here and there fills the bill quite nicely for less used sizes.

I use Scotch 77 spray adhesive for laminating, sharp new xacto blades for precise heavy cutting in cases like this, and sharpie markers to black out the sections that will be in the light path. I have not tackled a Vivitar (I had one for a short time, gifted to me, decided I did not need a fifth enlarger and sold it for a song at a photo swap show.

I have made neg holders for quite a few different enlargersthough over the years. Usually for more obscure film format sizes when printing older negatives from relatives who have passed on.