The UK Hyundai Coupe Club

Possibly, is the front one for looks? Or protection? Most people use some form of mesh or grill.

Is 40mm gap between the 2 sheets gonna be sealed/airtight?

You can do what you're planning, assuming you have the right kind of specs for your speaker. Look up 4th order bandpass boxes - the front section needs to be a specific volume with a correctly sized opening (although that can be a port, or passive radiators). Response tends to be improved at a particular frequency but crap at anything else. They also have to be pretty large relative to a sealed/ported enclosure (a properly designed one for a typical 12" sub will take up the guts of all your boot).

If you want something nifty I'd just do it completely stealth (i.e. carpet the grill)