I figure someone here knows the answer:

I need a "Remove Before Flight" type of plug for open fuel vent tubes. In particular, these tubes are about 5/8" in diameter. I envision some sort of rubber/squishy marble-sized ball with a visible flag.

Yes, we could probably make it ourself, but figure someone has good one on the market that won't have the flag come off and leave the ball inside the vent tube... which I'm sure will happen at some point if I make it.

Standing by...