If you had a battery tray, you could use that to help locate the holes.

I could take a few measurements of some relative distances of at least one hole.

With that one hole drilled, you could then eyeball the battery tray with respect to the center diff and any other measurements from known components (I can provide any measurements you'd like).

Obviously, the ECO chassis is the easiest way to do it. Not sure where you race, but perhaps a fellow racer has a worn ECO chassis laying around that you could borrow. Or even allow you to use a chassis that's in use, just disassembled.

Let me know if you need some measurements. I'd be glad to take them, with photos, and post them.

Jim