I modded a flashlight that work with 3xaaa and i needed an adapter. I found the solution near my mouse!
The mousepad (3mm) works great as adapter (and usually you always have an old mousepad that you don't use anymore). You only have to do a rectangle with one side of 5,9mm and the other one with the same lenght of the battery compartment. Then put it in the battery tube. The black side of the mousepad held in place the "adapter" and the other side is slippery and allows an easy insertion of the 18650.
I made some photo but is really easy so i decided to avoid them.
I know is a "silly" solution but maybe someone need a very budget adapter and don't think the solution is really easy.