Cheap NiMH D Batteries from AA NiMH and case adapter

I have no affiliation with this store, but I did buy some of these cases.

They work fine, but it took me a few minutes to figure out that a battery installed singly goes to one side, not in the middle of the case.

Two batteries fit side by side of course. These are $1.83 as of this writing and let you use AA batteries in equipment that takes D size.

Store link.

Pros:
Cheap D batteries, in a price and capacity you can choose by purchasing appropriate the appropriate cell. Or you could use AA non-rechargeable from the Mega Pack you got at Costco. Want more capacity? Use two cells.

You just need the right form factor, so this is your product.

Cons: The lid that holds the batteries in looks a little flimsy. A screw type retention would seem better, but the latch hasn't broken in light use over a few months. If you just install the batteries and leave them (rechargeables don't need to be swapped in and out if your recharger will charge D size) then it should be fine indefinitely.

Remember, NiMH cells are 1.2V vs. 1.5V for standard batteries. Your gear may be sensitive to this. Roomba walls for example gripe about the battery being low if the cells are not fully charged.