Sorry Gary___W, lots of posts from the time I started writing up my response. Here is my two-cents worth:
Another way to do this for transmitters requiring 12v is to use a not-so-new 3S lipo that still has plenty of power but a little-too much resistance in the pack for the higher-demands of motors. The power draw for the TX is so low that it doesn't matter unless the resistance is way too high; in that case get out the clippers and salt water to kill the pack like you should have done long ago!
I have the FlySky FS-TH9X
radio; it is a good one! I took one of my older 1500mah 3S lipos that fit the battery compartment. This is a "keep trying your batteries until you find one that fits" check; only one of my 3S lipos would fit. I had another of the exact same pack had slightly-larger dimensions and wouldn't fit, so be prepared for lack of uniformity in size. Cut the power lead off the AA battery carrier and added a male Deans connector. Hot-glued the circuit board connector back on to ensure it would stay put. Perfect fit, I can fly all day on this battery and recharge anytime with my lipo charging system.
See the attached photo for how I did it. Take note of your polarity, it might differ if you have a different radio. In the case of this radio just match red to positive, black to negative. I also use a rubber band to keep the battery from sliding around; this keeps it snug in the compartment. I think a 1000 mah 3S would also work no problem (I have one but it wouldn't fit the battery compartment), not sure about anything smaller.
Good luck, this was no big deal to do!