I picked up some 6V video cam battery packs that were on clearance from the local Radio Shack. Cost $1 apiece. I found a YouTube video on how to connect the NiMH, LiPO and other batteries by soldering the individual cells into a usable pack. The cells are 1.2V nominal apiece and 10 in series charged up to 13.8V FB. Rated at 2700mAH per 6V pack.
I pried apart the plastic cases and put them in line, then taped closed to avoid shorts and provide physical support. I use it to power my MFJ 259-B and a Weber Tribander for QRP backpack ops. The only caveat is that you should use a really hot iron to minimize heat transfer to the metal battery case. I like to use plenty of flux and would recommend that highly.
Each cell was about 0.608 V when initially checked. I guess they are shipped that way but it did not take long to get them up to full voltage using a 6V, 6A charger. That’s a pretty hot charge so I only left the pack on a few minutes and checked for over- heating, but none found. NiMHs are recommended for fast charge according to web info. Just watch for over-heating. I now use a 1000ma wall wart. Time will tell on how well they hold up. But for $2 and a little cold weather shack time I figure I can’t go wrong.
Here’s a slide show of the whole operation, which took about 2 hours of casual shack work.