Here is a workable solution. I have had the same problem with the battery covers on my two Aeris Atmos AI computers, made by the same company as the Aqualung I550. When my Atmos AI battery cover coin slots broke (same problem as your Aqualung battery covers), I was buying the Aqualung covers and heating them slightly to fit the Aeris computers. Now, that I can't find the Aqualung battery covers either, I put a large glob of PC Super Epoxy on the battery cover, masking the entire coin slot. As the epoxy starts to dry, I reshape a coin slot, or more recently make a large screwdriver slot in the top of the epoxy mound. When that stuff dries, it isn't coming off the battery cap, breaking or cracking. I switched to the screwdriver slot to avoid having to deal with a coin and pliers. Both of my Aeris computers are fully functional with no leaks.
This PC(Protective Coatings) Super Epoxy is really high quality, industrial strength epoxy you can get on Amazon or at some hardware stores. This epoxy is great, and if you ever need to repair rotted/damaged wood, their PC Woody Epoxy is phenomenal as well.