We have customers who have had luck with back plate equipped Ovation models. Otherwise there's nowhere else for TWA to sit flat on an Ovation.
You can find a PDF, printable version here!
I've an Adamas W597 which has the round covered opening with center screw in the bowl (body rear).
Do you have instructions how your amp would be attached to such a guitar?
Can it be placed on the top instead of the back of the ovation guitar?
We wouldn't recommend installing on the top of a guitar. It would be prone to lots of feedback, and will significantly dampen the natural sound of your guitar.