an expensive alternative....
If you're already flying and want an easy install (but pricey!) solution to the air leaks, try a rear seat stick boot assembly from Classic Aero; they have the vinyl ones priced at $191 for a front/rear seat pair... perhaps you could ask nicely to buy just the rear seat boot?
The rear boot has a vinyl/sheet metal cover that hides the whole rear seat stick well and blocks out any airflow. For me, a bonus safety feature of this unit is that since it covers the open stick/control column well, it keeps stuff from falling down there and getting lost or jamming the flight controls... easy install too, took all of two minutes to put in.
Yep, it's expensive, but quick and effective.