Another way to hold the tstat in the housing is to loop a rubber band around one of the bars of the tstat and bring them through the housing so that the tension holds the tstat in place. I use a pen or pencil or whatever and feed that through the ends of the rubber band so that holds against the end the hose goes on. Haven’t had an issue as long as the housing is flat.
That will seal, with rtv, without issue. I've sealed way worse.
But like stated above, the replacement is cheap.... just don't buy a dorman, I got 2 that were very warped and one that the bypass hole didn't line up at all
I've found that the thermostat gasket sold for our cars doesn't line up completely with the bypass hole. Now I make my own gasket using the thermostat housing and lightly tapping the gasket with a ball peen hammer to leave a perforated trace. I then cut it out with an exacto knife or raxor blade. I use tan paper gasket material they sell by the roll in the sealant section of most local parts stores. I use the same to make throttle body gaskets and gaskets for a bunch of other stuff. Quicker, cheaper, and better gasket results.