I've made a few for my washer-less swords that slip over the blade by tracing the scabbard mouth on an appropriate scrap of leather glued to a piece of red felt. I then cut the outline with a sharp scissors, and using a hole punch overlapping the holes to cut out the slot for the blade. For those with the washer Under the tang shoulder, only disassembly will let you replace it properly. I do not do that. If it's loose, it stays loose…
This is (or was) a sword outfitter in London that did sword, grip, & scabbard repairs and also sold washers, can't find them online any more tho.