If the holes in the playfield are left empty for too long, the wood can swell because it is unpainted/sealed .

Chuck the inserts from the inside, on a lathe and turn them down (someone you know has a lathe), take 5 seconds each.

Or you can make some correct size holes with a forsner bit in some acrylic and then use a router with a pattern bit, to perfectly route out the holes.

If you force un-jeweled inserts into too tight of a hole, they tend to quickly cup.

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