Next time you find yourself sitting around the campfire with your trusty pocket knife handy, grab a branch and make yourself a spare (or replacement) rod cap.
As with all things tenkara, take it slow and easy, use a gentle hand.
Make sure you don't try to force a peg that is too big into a hole that is too small. You could possibly damage you rod. You also need to be sure not to try to force the cap too deep into the rod, remember there are rod sections down there. If you have your current plug handy, you can use it as a template regarding length and diameter.