Used to run my KX1 on Eveready AA lithiums. They would last forever, like months. For a voltage that will get you in the 4-5 watt power out, just get an 8 slot AA holder and load it up. These lithiums have the highest watts to weight of any battery you can find either primary single use or rechargeable. They start at 1.7 volts when new, so choose whether you want 7 or 8 of them. John N0EVH
Or make a case change like I did. It worked for me and this ham also.