If you don't intend to carry the rig too far (ie. just to the beach, on a boat etc) you might just
look for a battery booster pack. Automotive stores and places like Walmart etc carry these
and often they have them on sale at very reasonable prices.
You would probably want something with a Sealed Lead Acid battery in the range of 12 Ah
to about 22 Ah. Most of these packs have 12V accessory plugs (i.e. cigarette lighter adaptors)
but you could disassemble the case and add powerpoles. Beware that 12V accessory jacks
are not designed to handle more than 10A so you would need to run the rig at 20W or less
which should be fine. The advantage of the booster pack is that it already has a case / handle
/charger and in a pinch you can even use it to jump-start your car ;-)
P.S. I have a 22Ah model that I keep as a backup battery for my QRP rigs at my cottage and in
addition to the 12V accessory jack it also has a 5V USB jack which is useful for charging cellphones, iPods etc.
It is a Yaesu FTM-350ar, and those are some pretty sweet cooler/radio rigs. Sorry I guess more info would have helped. I want to be able to carry my unit with a dual band antenna out on a boat, or beach or just whatever.
Josh(still waiting on my call sign)