The first thing I would do is clean clean clean the entire work area within the car.
A clean work area greatly reduces the chance of incorporation of dirt into those sensitive, and expensive thermostat housing threads.
Remove the hard lines from the top of the housing and clean clean clean the threads on both sides.
If you have noticed crud in there already, it needs to be cleaned out.
Then, don't go back to the shop that fitted those hardlines ever again.