Summary

I found 2 rechargeable NiMH D batteries for a project, but all I had is a small AA - AAA charger. I wondered if this Rayovac AA charger would charge the big Ds. Yes it will.

My Energizer batteries are rated 1.2 V and 2,200 mAh. In storage for a year, the battery voltages began at 1.20 and 1.21 V.

Plugged it in for a couple hours. Voltage is now 1.42 V.. Batteries are not even warm. I think a D has 8 times the capacity of an AA, so charging this way might take some time. (Came back in 6 hours and the light was off. Cooled, finished voltage 1.41)

Wondering how to do it, I designed the adapter in two parts to give me options. The upper and lower parts were eventually hot-melt glued together, but might be friction welded (link) or glued, or perhaps modified and printed as a single piece?
https://www.thingiverse.com/thing:1807093 (welding)

As for electrical connections, I chose stranded wire, and poked holes where needed. A touch of solder here and there can help make electrical contact. The heat also melts and secures the wire to the plastic body.
Strips of heavy aluminum foil might work. Whatever is used, the connections to the charger should be tight and reliable.

All these chargers are slightly different, and you may need to modify this design, or custom make your own adapter. But I do think any AA charger will charge the larger C and D batteries.

Print time on an Afinia H479 was 1h 30min with 0.3mm layers, medium fill, medium speed, with raft. Material 38.5 g ABS

These files are SAE (inches). If your software imports impossibly tiny shapes, it thinks the STLs are Metric. Just scale them up by a factor of 25.4 (in X,Y and Z) and all will be well...