http://lockwhenlit.com/products.htm

Rob Anthony sells them. On his products page in the above link.

https://www.lowes.com/pd/Hillman-1-5-Volt-Alkaline-Portable-Battery-Pack/3478223
Could get one of these and short one slot...
All of 65 cents.. plus tax...

https://www.marcospecialties.com/pinball-parts/RBH073

I always like to install NVRAM (desoldering/soldering required on WPC) on all games I work on, including for others to protect the games. https://www.pinitech.com/products/6264_nvram.php

Do you need a diode there or just a straight short?

Diode on games that normally recharge the original batteries.

It doesn't hurt to put a diode on all games instead of a jumper wire anyway, in case the blocking diode on the board shorts you won't try to charge alkalines.

Since there are normally 3 batteries in the remote holder at 1.5V per cell for 4.5V total, 4 batteries would be 6V.
So, you can just short one cell slot with a wire or other means and just use 3 of the slots (no battery in the 4th slot).
It might also be possible to just move one lead to a point where only 3 cell slots are used on the assembly and just ignore the 4th slot...
This is not quite as eloquent as having a battery holder with 3 slots, but at 65 cents....
You can decide what works for you...
You might also consider just forgetting about AA batteries and use a lithium flat 5V cell instead.
I think there are some mounts available that you can use to replace the 3 AA cell mount.
I see a lot of people doing that to avoid acid problems with AA batteries.
Then, you don't even need a remote battery holder.