For something compact enough for a Novamill, you'll probably have to build something yourself.

It's worth looking at the SkyfireCNC options (If the info isn't on their website, I do have the PDF documents somewhere).
I also got as far as some sketches for a compact big bore 4th axis for my Triac, and my conclusion was a suitably adjustable worm and wheel was going to be the best compromise. Custom made also gave the option to mount the motor at a suitable angle, so as to minimise overall height of the unit (probably about 30 degs off horizontal, so the motor sticks out the front, but doesn't mean the axis has to sit higher to give clearance for the motor, or having the motor sticking straight up)