An easy-to-build adapter that allows you to use AA batteries (especially rechargeables!) in place of D cell batteries in many devices.
Step 1: Cut a Piece of Dowel, Drill It Out Most of the Way
Drilled out a piece of 1 1/4 inch dowell to about 1/3 of an inch from the bottom, to make the dowel + AA battery the same heigh as the D cell. 1/2 inch drill bit is just a bit small. I think a 5/8's would work best with a drill press. Without a press, a 9/16's would probably be best.
Step 2: Pound Tee Nut on Bottom, Install Screw.
The "inspired" part. Drill about a 1/4 inch hole through the bottom. Pound the Tee Nut in place. Screw the short round head slotted machine screw in from the inside and adjust it to make the AA height 'just right'.
Step 3: Use Them!
Here's two of the adapters in our Roomba's "virtual wall" that requires two D batteries. We can now use AA rechargeable batteries and save $$$.
A drill press and a little sandpaper and these would have turned out much more professional looking. But they are functional as is, if a bit raw. You can make 8 of these for less than $7, no shipping.
Although such battery adapters are available on the internet, I was not able to find any locally.