Ok.... with regards to " making a mess" ..... does your car not have the arrangement where the access to changing the thermostat is a compressible rubber biscuit which does not use and sealer ? Thus it should not be any mess or trouble to open it up and look or replace it....
Did you check your new thermostat in hot water to see if it was opening up at the prescribed temperature ? What brand did you install ? Does your system have the seperate ' burp pipe' built in ? Does your thermostat have a small hole in the flat part ? If yes, is it at the top when installed ?
Why did you replace the thermostat ? IE, was your car overheating prior to this current thermostat being installed ? Was anything else worked on or changed at the same time this thermostat was installed ?
And , YES, some people have bought Brand new thermostats which were Bad out of the BOX..... a truely disgusting situation when it happens....