Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4769764.
18650 Cell to 2x C-Cell Battery Adapter by RebelTaz
A friend asked for this and I saw a couple of these on here, but I didn't like the designs for one reason or another. So, I submit for your approval (or not, I don't care) my entry into the battery adapter arena.
You will need three additional pieces of hardware - a 1.5" #6-32 screw with a beveled head (common in the United States for use with electrical sockets and cover plates), a #6-32 nut, and a 1" fender washer with a 7/32" inside diameter hole. The nut is placed inside the adapter and pressed into the recess while the screw goes through the washer from the bottom and is threaded into the nut. Easy peasy.
NOTE Due to either a Thingiverse bug or (more likely) shadowbanning, my models no longer show up under the [Newest] listings. If you want to see more of my models, please be sure to 'follow' me. If you print this, it would really help if you'd post a 'make' so that others could find this model.