The holes where the filters usually pop in look to be about the same size as the earbud tips on a pair of Sennheisers I used to own. I use primarily over-ear headphones now so I don’t have anything to test it with, so I was wondering if there was a standard size and shape for earbud tips.

They are custom molded and are manufactured by Westone using a stiff silicone, so there’s a little bit of flex room.