I have several old, plate holders. The ones made to hold dry plates, in general, will fit my modern cameras without much trouble. These often appear on ebay with film sheaths available. The older wet plate holders are all too wide to fit my modern cameras. The true wet plate holders are hinged on the side to allow the plate to be put into the holder from the back.
When buying a plate holder, I e-mail the seller and get exact size of the holders before buying. I have thought about trimming the old holders down by taking a 1/4 inch off each side, but haven't had time to do it yet. It should work, however. I just haven't decided if I want to ruin a 100+ year old holder.
By the way, most dry plate holders can be used for wet plate, but they are harder to load and a lot more work.