Balancing a Pneumatic wheel

[F22Rapture]

We've got 2 6" Andymark pneumatic wheels on our shooter, and we were having some major issues with vibration due to the air spigot throwing off the weight distribution. We removed the innertubes entirely and filled them with bubble wrap which helped some, but it still vibrates badly because the tire doesn't seat particularly well without inflation. For teams that are using pneumatic shooter wheels, what did you do to balance them?

Also, did anyone find a different type of wheel to be superior?