You know how the software at Amazon and eBay likes to make suggestions based on what you're buying? "If you like this product, you might like THIS!" Sometimes their suggestions are good, sometimes the suggestions are so bad as to be really funny. This one MIGHT have been good....
I was looking on eBay for some other watch parts when this popped up: PVC Watch Strap Retaining Hoop Loop Rubber Retainer Buckle Holder Ring Keeper | eBay
They come in black or white, and they come in four sizes based on the INSIDE WIDTH of the loop, which sort of suggests that the seller and/or manufacturer know what they're doing.
I haven't bought any yet, and don't know anything about the seller (who has 39,782 mostly positive feedback over three years) but for $1.40 which includes shipping to the US, Canada, UK and Australia, it seems like they might be worth a try.
The question of where to buy aftermarket "strap keepers" comes up around here quite often and the answer is always "Casio doesn't sell them individually" and "use an O-ring or an elastic ponytail holder instead" so I thought I'd mention these!