Depends on how involved you want to get and how permanent you want it to be.
At one end, stencils and an El Marko will do an adequate job (though will need to be refreshed on occasion.)
At the other end, you can get die cut inlaid panels in PVC or hypalon that get glued on like a patch.
In the middle are a whole range of options, like this:
BOATNUMBERPLATE Registration Plate Numbering Kit | West Marine
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