FX Maverick Magazine O-Ring

I use a glue which is less permanent than cyanoacrylate (crazy glue) - you may want to change that o-ring in the future due to wear.

I also have found that these are pretty cheap and easily available, and work in FX magazines, with just a toothpick end of rubber cement to hold them in place. For the time you drop one out but don't find it on the shooting table.

https://www.amazon.com/gp/product/B07MNJX3ZB/

5mm OD x 1mm ID x 2mm width (thickness). I think that the OEM FX o-rings have a 1.85mm width.