Making a hump pack isn't too difficult. Just get 5 batteries. I used AE size NiCd's on the last one I made. That's a pretty typical size for these packs. Best thing to do is to measure the space you'll be mounting the pack into just to make sure you don't make it too big. AE size batts are 28mm long so they fit good in my Maxx. If you look at that table that Dave put up, you can see the size difference in batteries. You'll also need a connector that will plug into your switch or receiver. And don't skimp on the soldering equipment. Good iron around 75w with some decent silver solder should make it easier. Arrange your batteries in the shape you want and then use a little shoe-goo to stick them together. Take your time soldering them. It takes patience to get them to stick. Rough up the solder pads and that should help. To make it easier you may want to buy batteries with the tabs already on them. Then just go at it. When you're done, shrink wrap the whole thing and it should work fine.