I assume you also strap down all suitcases, shopping etc every time you have them in the car. Believe me I'm well aware of what damage can happen in vehicle collisions and roll overs.
Actually, I do secure heavy luggage when travelling on holiday, and the dog too, who has a harness. As for shopping, I don't drive everywhere with shopping in the boot, nor are the contents of your average shopping bags going to kill you at the speeds most people do on local roads.
I'm not trying to be smart, I'm just pointing out that putting the boot mat over it does not make it secure. If you're happy with the way you have the wheel secured, that's great - I'm just pointing out that having a 15Kg lump loose in the boot is not safe in an accident.