WELL... I looked at the OP.... and AGAIN..... , you have outdone yourself! Modders work harder than us idiots that just build from stock!!! Your creativity is as always... AMAZING! You absolutely, positively affirm the saying that "Where There Is A Will, Threre Is A Way!"
Thanks for sharing Justin!!... Most days in my shop I use one thing or another that I have learned from YOU...sometimes two or three!!!
Have you considered building a drop-out battery carrier? In this case with the shorter cells and loading it from the rear the front contact board would have to be a cloverleaf shape to clear the ID-shape of the stock tube, but the rear could be just plain round as it wouldn't need to pass through the flutes. With a single fatter-than-normal threaded standoff (I've seen it in 1/4" hex, would be plenty strong enough) thru the center to tie them together. That stuff is available in brass, so it could be soldered at the ends which would be more secure than a single screw with one central standoff.
It was one of those days. The kind of day when the wife just goes and hides with the cat. That mistake was just one of several "issues" that morning. It doesn't make any diffference really, as long as I don't forget and hook up the wires wrongrightimeanwrong, when I have forgotten in a day or so.