IJ. That Hire Contract may or may not "stick" and may need a darn good lawyer to do the drafting. (How can I possibly use the words "good" and "lawyer" in the same sentence?)
Where would you stand if any accident was not the pilot's fault? For example, engine failure, unattended damage on ground, etc. And what is it going to cost in legal fees etc to "extract" the excess from the hirer?
I'd be inclined to start with a car hire agreement and work from there.
Indeed, the only solution may be to take the insurance excess up front as a "deposit" on the basis possession (of the brass) is nine points of the law! Or alternatively, have an additional daily rate premium paid in advance by the hirer, in lieu of, or to reduce the excess. Most insurance companies will come to the party on that one.