The average flashlight battery is i.5 volts. That goes no matter if it is AA ,C or D cell.
Most radios take about 8 - 10 D cell batteries.
Do the math 8 x 1.5 = 12.

That means you can run your radios on a 12 volt car battery or , my favorite , a lawn mower battery.

Simply attach a set of leads in the battery box and attach them to your 12 volt battery.
On the older radios ( no warranty) it is a simple matter to open the case and solder wires onto the positive and negative terminals for the battery box.
Splice them in don't remove the wires from the battery box terminals.
Add a quick connect speaker jack onto the outside of the radio case. Hook the wires to the speaker jack Put the case back together.

You can still use the D cell batteries as well

In use it is a simple matter to run a set of wires from the speaker jack to a 12 volt battery.

I run my trans oceanic this way when away from a 110 source.

It saves my D cells for my flashlights.

Plus 12 volt batteries are easier to charge and last a lot longer.