Underwater Cities Disc Stacker

This is for folks with the double-layer player boards that come with Underwater Cities: New Discoveries. There may be some other way to get them. 

Tired of having your stacked city discs go flying any time you bump your city, fat-finger picking a token up, or even sneezing too hard? You're in luck! If you upgrade a city's disc, you can drop one of these into the hole, and drop the original disc in the top. Nice and snug.

If you don't have the dual-layer boards, but still want something that won't go flying, you can just flip a stacker-with-a-hole over and put it on top of your discs. At least they won't go rolling around anymore.