You are not the first, others have reported stripping out the tensioner bolt as well.
The fix is to get a metric HeliCoil Kit, tap it out with the correct size HeliCoil tap that comes in the kit and install a HeliCoil thread repair.
HeliCoils are much stronger than the original aluminum threads.
If it already had a HeliCoil installed and the the bolt was over torqued and pulled the HeliCoil out, then your only option is to get an oversize thread insert like a timesert (Model Number: 1812) and install it in the cam belt back cover. They are not cheap, but will work in extreme cases.
Other option is to locate a good used cam belt backing cover that is not stripped. Timing Case. OEM Part Number: 05159211AB
Tensioner Bolt PN: OEM Number: 5015031AB
DO NOT OVER TORQUE: (Torque Spec: 22 ft/lbs)
