tail tie down
An old rule of thumb (the practical kind you don't find in modern textbooks) in prop design is that a well-designed prop will produce about 4 pounds of static thrust per HP. That should get you in the right ballpark for figuring how to secure your plane.
For various maintenance tests I have tied my tail to the trailer hitch on my car (with a 215 HP Lycoming and constant speed prop---not an RV--12) and to tiedown rings at the airport, all without any problem. On the RV-12 I have installed an eye bolt into the tail tiedown fitting and leave it there. It is surely strong enough to do this job.
RV-12 #120768, first flight Sept. 2016
344 hrs as of 8/27/2021