Idk, might be just me but I think this idea has some creativeness and clever engineering but come on, you can buy a full set of new mounts on ebay with a lifetime warranty for 100 bucks. Thats what I did, no rattles, no noises, solid mounts, and you have a peace of mind rather than dealing with it again in the next year. I know 100 bucks is a hundred bucks, but im sure all of your cars have over 100,000 miles, so I would say do yourself a favor and just get new mounts, its finally time. And it probably easier to install those than the time it would take to cut and fabricate a hockey puck. I mean the window weld sounds like a temp. idea, but if your putting hockey pucks in there you might as well just do it right. Just my opinion, but still give credit for the thought behind it. But in terms of material, yah its a more dense rubber, but its an overall different polymer as well. Engine mounts insert use a copolymer (isoprene and isobutylene) where as hockey pucks are just vulcanized rubber. AKA vulcanization is utilizes sulfur to bind the C-H chains togethere resulting in the extremely dense strong material. The problem is with using the puck is that it lacks compression qualities for going over bumps, etc and thus could result in higher stresses on all the mounts where as the engine mounts materials use polymers specifically designed to undergo extreme absorption taking extreme stresses off the mounts themselves. Still your call....