Lantern bateries come in 6-volt and 12-volt versions. They ain't cheap and they
often derate by setting on a store shelf a long time.

The little gel cell, float-charged by an ac or dc adapter, or solar cell, will
work fine. Any of these should provide current at least equal to the duty cycle
of the load current. Your radio requires 500 ma. So, if you use the radio an
average of one hour out of every ten, then the charger should provide at least
50 ma.

A correction regarding your suggested series connection of batteries: Connect
the positive terminal of one to the negative terminal of the other. The far
ends then would connect positive to positive and negative to negative on the
radio.

Bill, K5BY