I havent tried this on my tc yet, but it worked very well on my old car.
http://importnut.net/motormount.htm
Just fill one mount at a time. I only filled the two lower mounts on my last car and not only did it work perfectly, but there was not much increase in vibration inside.
I did not use window weld though, I used some industrial urethane, but I cant find it now. I have heard good things of window weld though.
I would reccomend just filling the rear lower first if you plan on doing this. Then you can fill more if you dont like it.
If I ever get around to this project myself, I will probably have an extra mount and can make them for people with a core charge.