I would make a blank plate for the existing instrument hole, drill holes in it that to match the existing hole pattern in the D3 cradle, and attach the D3 cradle to that.
Alternatively, I would fabricate a clamping bar that's slightly larger than the 2 1/4" hole, maybe a piece of 1/2" or 3/4" wide, flat aluminum bar stock. Drill a hole in each end of the bar stock near the edge of the 2 1/4" panel hole diameter. I don't have a cradle to measure, but if the diameter of the recessed (dimpled) holes for flat head screws are close enough to fit in the 2 1/4" hole, I'd use those to put a couple of screws through the cradle and the bar stock, with nylon lock nuts on the back. Feel free to laugh at the artwork. I don't have the tools to make a proper diagram, and it's been many, many years since my engineering drawing classes in college. And the screws would obviously be shorter than depicted here.
If the existing recessed holes in the cradle are farther apart than your 57mm panel hole, you might need to get a bit more creative. You could drill new holes in the cradle, but you may have to recess them to allow the D3 to fit flush in the cradle (I don't know how much room there is).
These are just first thoughts. I might come up with something different given more time, and I'm sure others here will have their own ideas that might be better.
And if all else fails, there's always velcro. ;-)