Hi Mark, I don't have the ETI box but going down through the bottom is probably the way to go. I'd put a grommet on the hole to protect the cable from the fg edge.
When I was doing similar work I took the opportunity to improve the overall ground reliability. If you're going to be disconnecting your ground cable you might consider doing the same mod if your ground cable is also attached to the frame. Maybe not, maybe that's only for installations that don't order the factory inverter.
At any rate I didn't like the multiple ground attachment points on the frame, each with a potential for becoming unreliable due to corrosion. Attached is a photo of my mod. I tapped the frame and installed a s/s stud. All grounds go to it. My view is lug to lug contact is much more reliable. Each set of brakes was also grounded to the frame so I installed the white cable to each of them and forward to the ground stud. The whole assembly was then coated with liquid electrical tape.
So, if you want to make your project more work, that's how to do it.