Radio Shack has 3 battery AAA holders; they also have the 4 battery. The problem is you get 6VDC with 4 AA or AAA batteries. 3 give you 4.5VDC (appropriate for the Williams boards). I have always used the rechargeable battery packs you get for cordless phones for Bally boards. They are available almost anywhere and they are 3.5VDC like the original NICAD pack batteries. Like BD, I either solder on my own remote wiring connector (with a quick disconnect) or I install a two pin cage terminal block.
I do mine 100% of the time. I never leave the batteries on the board. I think that was one of the worst design ideas the original manufacturers had. In fact, the first thing I do when I get a machine is remove the battery. 2nd thing, throw away the pinball (unless it’s brand new) and install a new one.