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.
Step 2: Finish the Paper Roll
When the glue has dried, the roll of paper will be firm like cardboard. Add a wooden plug to one end of the paper roll. Use a 3/8 inch dowel for this. Cut 3/16 inch from a dowel rod. Smear it with glue and insert it into one end of the roll. Allow the glue to dry thoroughly.
Get a thin brass rod or a short piece of #10 bare copper wire. Drill a small hole in the center of the wooden plug and press the copper or brass piece into the hole.
Place the roll over an AAA battery. File the brass or copper piece until the length of the AAA battery with the paper adapter is the same length as an AA battery.
Step 3: Charge Some AAA Batteries
In the photo you can see how the AAA battery with the rolled paper adapter fills the same space utilized by an AA battery. This photo is for illustration purposes only. In actual use it is unwise to charge different types of batteries in a charger like this. Also, this charger calls for either two or four cells charging in it at the same time. You will want at least two of these adapters. If two are charging, place both batteries in the same side of the charger, left or right. Check the directions for your charger.
I now have a larger charger that can handle AAA or AA batteries. But, this small charger is light and compact. It still makes a good choice when traveling.