I had an AA NiMH charger for my digital camera. A couple of years later I had a couple of devices powered by AAA NiMH batteries. I wanted to use the charger I already had, but it was made for AA batteries only.
Step 1: Make an adapter from paper and glue
Use a piece of junk mail to roll an adapter. Cut a strip 1 7/8 inches wide from the long side of an 8 1/2 x 11 sheet of copy paper like you get everyday in your junk mail. Begin to wrap it around an AAA battery. Do not wrap it too tightly, but leave it just a little loose. Smear wood or paper glue in front of where you are rolling. The black pen line shows about where to begin. Spread the glue out very, very thin and roll forward. Apply more glue, smear, and roll until all paper in the strip is rolled around the battery. Apply some cellophane tape to hold the roll while the glue dries. The diameter of the roll will be just a little less than that of an AA battery. You could add part of another strip of paper to make the roll exactly the thickness of an AA battery, but it is not necessary.