battbox-Base.stl IS NOT INTENDED TO BE PRINTED, JUST TO SHOW HOW TO MODIFY YOUR EXISTING PART.
Drill an M3 hole in the center of the hex hole and remove extrusion around it and the back wall.

This battery box has a vertically sliding door with a screw to keep it captive.

If used with the Res-V3 or B36-3 electronics:
It also has a mount for a 2 position momentary switch which will be pressed when in low gear, allowing for controlling two separate external devices using the slider on the remote, my own use case will be two winches for raising/lowering the boom and reeling in/out the hook of a wrecker addon im currently remixing.

The battery box should fit the B24 as it has the same spare tire mount but i cant vouch for if the function switch will work, it might but will depend on if its position, relative to the gearbox, is the same as on the the B14/16.

Ive included the specs on the momentary switch im using in case the one i used: https://www.amazon.se/dp/B07YDFH7H3?ref=ppx_yo2ov_dt_b_product_details&th=1 isnt available to you.

To mount the battery box one will need to modify the tray in which the original box sits, see battbox-base for an illustration.

6mm M3 and nut for mounting the battery box.
The momentary switch, door retainer and connector clamp all use 6mm long M2 bolts.

Ive also include a very simplistic illustration of how to wire the two external devices, motors will probably be the most common, the slider on the transmitter will cause either a positive or negative voltage to be sent, depending on direction pushed.
You make sure whatever you have connected can deal with negative voltages if you wish to avoid risking damaging something

EDIT: Took it apart to prime it so i snapped a clearer picture of how the momentary switch is placed.