Sonoranwraith-I may try the vaccum but if it doesn't work the air bubbles I have are not enough to be concerned about if the other tires turn out this good.D8R-there is many different hardness materials to use and also rigid products. Here is a link to the company I used and you can see for yourself-
http://www.smooth-on.com/index.html
I used the vyta-flex-30 urethane rubber for my tires and they are flexible. You can see the flex here. The wheel of course will stiffen it but I think it is about right-
