I have lots rechargeable AA batteries, and a few things that require D-cells. This will let those AA's work in the D-cell applications. (LED camping lantern, mattress pump etc.) and the AA's are small so you can carry extras and replace them as needed. (Yes they might not have the current capacity of D-cells, but it works pretty well for what I needed.)I designed it to use the 1-inch brass paper fasteners. You can buy a box of 100 at an office supply store for around $4. 1) Print (took me ~50 minutes ord-bot 0.35 nozzle) 2) file the 4 inner supports to fit your battery. (2 minutes) 3) insert the paper fastener at the top, use needle nose pliers to fold the tabs out then back on themselves. If I have time I have a few improvements in mind, but these worked for the weekend. I just uploaded d-5 which loosens the grip on the battery. (untested) ...