How to waterproof my battery

My battery for my custom board just arrived, but I will not get the bord for a few weeks to a few months. Custom takes forever :).

The point is, I want to test the battery/esc/motor combo out.
The battery is 350mmx230mmx110mm, and I’m trying to find a temp solution to strap it to the back on my current foil board for some testing. Does anyone have any ideas about a cheap (under $60) solution to keep it dry?


Sport gear bag for boat , Jetski , paddle