So I bought a new set of bokuto (Tachi and kodachi) and nice leather tsuba, however they did not come with tsubadome (Which they would if I'd bought the default plastic tsuba.)

I looked for a tsubadome file, and found one, but I didn't like it much. I was looking for somehting simpler, but maybe nicer than the glorified rubber band that comes with the plastic tsuba. It's not like I don't have a bunch of them anyway.

For the picture, the only TPU that I had on-hand is black. I'd like to use a nicer colour, but black will do fine as a proof of concept.

The file is 10mm high, and that seems a good height. The bottom is flat, and the top is rounded. I recommend scaling in X and Y to be a tight but not impossible to fit size for your bokuto. The kodachi will probably be just a little smaller, though you ought to be able to find a size that works for both.