Roboticist here. Same desire. I really like the photos of the bare aluminum Reform Next. 
Maybe the surface finish can be sanded away by a car-repair-shop or similar?
The classic Reform trackball is certainly too large. Did you check the CAD files whether the Pocket Reform trackball would theoretically fit?
In maybe slightly related news I’m working on a trackstick mod for the MNT Reform keyboard 4.0: Another alternate keyboard idea - #5 by josch I’ve received a leftover keyboard 4.0 PCB without switches and plan to do the hole drilling in maybe a week or so. I also still have to find my bag of kailh switches… should be somewhere…