The seal string that holds the backplate to the back of the mobo should stop shorting, but I use a cutout of neoprene and some electric tape to make sure
The metal plate also helps folk to check the rear heater is working ok- it warms up enough to tell
And to be safe I cover the backplate with electrical tape and use either seal string or a neoprene gasket - not becuase you need to , more as a precaution.
depends what the matting is made of- there has to be an airtight seal to stop condensation. If its closed-cell neoprene, ya canuse that as part of the seal. If it isnt, I wouldnt risk it