I have several 6 volt lantern flashlights kicking around the shop. However, I never use them because I generally baulk at the $7 they want for a lantern battery that can't be recharged.
But the other day I noticed a 4Ah 6 volt recharable lead acid battery at Home Depot, these are made to be back-up power sources for emergency exit lights and they cost between ten and twenty bucks. But more importantly they are very close in size to the 6 volt lantern battery.
To modify my flashlight I cut off the outer connector ring to prevent the possibility of the battery connectors shorting, then I soldered wires to the two connections and crimped quick disconnect lugs to the ends.
I added a second pair of wires to the lugs and soldered those to an RCA connector that I press-fit into the back of the flashlight. This connector is for recharging the battery. I reassemble the flashlight with a piece of Styrofoam filler to keep the battery in place.
A 6 volt lead acid battery will need a charger that delivers a bit more than 6 volts in order to take a charge. I searched my big basket of wall-warts and found one that would provide about 250 ma when connected to the battery and plugged in.
Since this is a 4 Ah battery, 250 ma is just about the ideal 20 hour charging rate.
My lead acid battery turned out to be about 1/4" taller that the 6 volt lantern battery and the flashlight head would not quiute screw all the way on. This was easily remedied by setting my heat gun at about 400 degrees and heating the polyethylene case until it softened and stretched, you have to heat and stretch the case beyond it's elastic point to retain the extra room for the battery.
Now I have a fully rechargeable flashlight and the lead acid battery will pay for itself in just 2-3 cycles!