Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3859709.
The original NEMA 23 Cap thing at https://www.thingiverse.com/thing:1725841.
NEMA 24 steppers are evidently just slightly larger than NEMA 23 so this is a NEMA 23 cap resized for NEMA 24 (thanks eastlight_jim).
So, I took the original model and used the 2D profile for the basic dimensions and then re-modeled the whole thing so that the bolt holes and edges would line up with my stepper's case in addition to a slightly longer cable cover that is moved about .5 mm further out so the cap is easier to install and the cables interfere less.
I've only remixed the "Middle" version of the cap because that's the only one I need for my application.
This cap is modeled to use M3x30 screws which leaves about 2mm sticking out to thread into the back of the stepper case.
Thanks to Mynasru for the original design.