I use a stainless steel washer I bought at the hardware store (I think it was a 1/4"). I ground down two sides and curved it so it fit the channel of the rim perfectly. I made a bunch of them at the same time taking about 1 minute each.
If that's too much, you can do a decent job by carefully de-burring the inside of the rim's valve hole, and sliding 2 of those fiber reinforcements school kids put on looseleaf notebook pages onto the tubes valve. Or use a paper hole punch on the middle of a small round patch, and bond it to the tube at the base of the valve.
It doesn't take a lot of complexity to adapt the valve and hole, so think creatively and improvise with what you have.
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FB
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