This holder is designed for the Dremel model 395, which has a ~47mm round body in the clamp area. The front mount is slightly tapered inside to allow the Dremel motor to just be slipped into the front mount until snug, and then close the rear clamp. The front holder ID is sized for the motor's stock 26.75mm diameter nose piece.
I printed this in both ASA and PETG at 0.2mm with 4 perimeter walls, 5 and 4 top and bottom layers, respectively, and 30% gyroid infill. You may need some supports depending on how you face the parts to the bed. (In later prints I laid the clamp bottom on its side vs. the green print bed picture.)
The screw holes are sized loose on the outer tangs and snug on the inner barrels, so that you just thread them in and no nuts required.
You will need two M3x12mm flat head screws and one M3x20mm flat head screw.
Also included is a fully parameterized Fusion 360 model for the design. The latch design (included) is imported/referenced by the motor holder model.
I borrowed ideas, such as the latch design, from Vegster's Dremel holder latched MK2 (but not nearly as sexy).
V1 Update: The V1 files make a few minor refinements:
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The author remixed this model.
Redesigned clamps for Dremel models 4000 and 395, and Ryobi model HT20VSK. Borrowed concepts from this design.