I have a 3d mag and use Kd's 3aa - d converters. Works great on eneloops. I also have 2aa - c converters. I don't have any c flashlights and would not buy any more c or d lights.
I think that many consider 3xC or 3XD as too bulky nowadays. Also the C- and D-cell format is not getting much love from the budget manufacturers. I own a 3C and a 3D now (both with XR-E), which are kind of budget (sub 20€ range) but the overall output/efficiency is not as high as latest technologies on AA or LiIon lights.
What sort of current draw? I found those 3AA-D adaptors from KD useless for 4+amps. Problem is, the better multi-AA holders with low resistance are ridiculously expensive. Some enterprising Chinese manufacturer could sell quite a lot of those at a decent price.
Fortunately the Fusion36 dropin doesn't need anything like those currents. From memory it draws about 1.8A with 6 cells. It works best with 4 cells or more. With a shortened tail spring and a bit of plastic pipe you can get 4 C cells into a 3D Mag.
For me, 3D is the upper limit for useful size. Beyond that, the lights are more luggable than portable.
Only about 0.7 A but batteries are not fully charged. I prefer using 2 18650's but it does produce a lot of light with 6 eneloops. A 2d would be my preferred host for this set up. But mine was stolen and its too expensive here to replace. I do like the "club' factor of the 3d. Doesn't really get any use when I have 50+ others to choose from.