I have a MG16 that i like a lot but was sick of the 3 tanks per 4 AAs. Here was my solution.
I had Cheapbatterypacks.com make me a 5 cell saddle pack. It is close to the same width as ths 4 AAS. When i put them in the battery hole the center drive shaft still has plenty of room.
Only modding i had to do was grind the AA cell alignment bars flat and trim some of the side wall of the holder. The 2/3aa cells are a little thicker than AAs but it works for me. I did use a little longer screws and 3 small washers between the frame and cover so the cover would not stretch too far before bottoming out.
I also ran the battery wire in to the receiver box so i did not have to pull it out for every charge. Now i flip the lid and there is the plug.
Pics

looks a little tight just check to make sure the screws don't back out but other than that as long as its rated for a five cell you should be good to go

Nice job with the custom battery looks good guys keep it up !!!!!