Completely waterproof (IP95)

matching the industry standard AMPS hole pattern

a battery bag from Rebelcell may be relevant

This is what can be stored in the Prison Pocket:

Railblaza NaviPack Navigationsljuskit Flyttrea Completely waterproof (IP95)Railblaza NaviPack Portable LED Navigation Light Kit The NaviPack Portable LED Navigation Light Kit is the perfect companion for small, steerable motorboats, inflatables or in an emergency. In a convenient affordable package, you get everything you need to meet the legal requirements in almost every country. Comes with three Railblaza mounts, you can use StarPort, SidePort or QuikPort. There are endless mounting possibilities. Dismantling when you are