BWD, what is "small temperature changes" ?
If you mean the ones we experience often, maybe 25 °C (45 F) temperature span then you are right.
As in your car it is maybe 25-26 °C (77-79 F), and the water right now in the winter here is 0 to 1 °C (32-34 F).
In the summer, a hot car can get say 45 °C (113 F), if it is 21 °C outside (70 F)
If water is warm about the same 20 °C (68 F) then it is also a 25 °C (45 F) temperature span.
Whether that is a small change in temperature or not, is of course relative, agree
Wouldnt the best sealing be with a "cork" of some kind in the sealing (like the air screw in our windsurfers for flying) so easy to drain ?