Cymbal felts.
I've been using the tennis ball beaters for the past 6 months and the right beater started falling apart from the inside. This lead to the ball becoming loose from the rod and it even fell off at one point. I tried to secure it with a washer but the hole only grew larger, eventually the rod moved to the edge of the ball. Because the rod was offset from the center, whenever the ball would compress against the bass pad, the washer ended up sticking out farther than the ball and bore a hole through the mesh head over the span of a single session.
Instead of replacing it with another tennis ball where this would most likely happen again anyway, I noticed I had a bunch of extra cymbal felts which almost fit around the beater rod. I wrapped the rod with some craft foam to fill in the gap and then stacked 4 felts on top of each other.
Couple things I noticed: it's just as quiet as the tennis ball beaters, there's less overly bouncy retrigger (for those who bury the beater), and after playing for 2 hours I haven't noticed anything that negatively impacted my playing. This solution is definitely cheaper than buying those official tennis ball beaters (especially if you're from the US like I am and are forced to pay exorbitant shipping fees), and easier than having to drill out your own mini-tennis balls.