Mounting batteries in a TAndem Island

Looking for suggestions
I plan to mount 4 50ah lipo so my tandem island it will be a tight fit I’m hoping to place them around mid length of the hull  they are intended for my motor guide xi3
I realize it’s more than likely over kill but I don’t want to be worried about ruining out of juice trolling or on a electric only lake going across
at distance

Not quite sure how to keep them stationary
And with the lipos in there I’ll. Be adding an auto bilge
If you have done this pics. Could really help

I used a garden kneeling pad,  cut out areas for the transducer, and battery. I used Lexel to glue down the pad and transducer. The battery pops out to charge. Caution! Lexel takes a long time to dry, and is extremely difficult to remove. you may want to try a different adhesive if you use this method. The use of Lexel was mainly for the transducer since it doesn't form bubbles.