I just thought of something else I didn't mention. Most all of the rubber gaskets that are made for applications have some properties that make them work best.
Lets use a valve cover gasket on a GM 3100 V6 engine.
The gasket on this application is all rubber. It fits into a groove machined in the valve cover so that when the cover is tightened down, the rubber cannot "squeeze" out the side. The groove makes the rubber hold it's shape, and therefore, do it's job. On the same engine, the intake manifold gasket (I'm using a Fel-Pro Permadry Plus for this example) is attached to an aluminum backing plate. There is rubber attached to both sides, and is not all that thick. When the intake is torqued down (not a very high torque) the aluminum plate again, hold the rubber in place. The fact that is glued to the aluminum makes it retain it's shape and not just "squeeze" out of place. There are countless other gaskets on lots of different applications that are made with this metal center plate.
I guess my point here is that a single piece of rubber used as a gasket for the rear cover, I fear, will just displace and squeeze out when you go to torque it. I think you will find that you won't be able to get the bolts tight enough without the gasket moving out of place. A paper gasket (such as the OEM one) will withstand the torque process and not displace when it is tightened as that material's "crush" strength allows it to retain it's shape. I'm not sure it will work at all...... but it may surprise me!
