I know my mtb, @MichaelWA’s MTB, and a number of others used an apex deck (rip apex) with pre installed threaded inserts in the deck which could be installed yourself on any deck+pelicase combo (tutorial+more info on that here) this is probably one of the cleanest looking methods but pretty inconvenient if you want to quick swap batteries, maybe check out the MBS Agent series, I believe they have a pretty refined quick swap system they they may be intending to sell aftermarket or could potentially serve as a starting point to design your own
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