My dentist made me a contraption that was custom fitted/designed to keep the tongue immobile so that it got the desired rad dosage. I do not think that my RO used this before, and it was a first for my dentist who fortunately made this overnight (to be used for the PET scan/rad planing and subsequent treatment). Now they are doing this routinely.
I also used cotton rolls but these were used to get my lips AWAY from the radiation field. That of course has to be ok with the RO.

Re scattering x-rays.
I seriously doubt that a plastic tray (or cotton rolls) will be able to "absorb" x-rays in the MV range!
The xray energy used for CT is in the "whimpy" kV range where you will get artifacts from dental fillings... if you did use a MV CT there are no artifacts. IMRT uses x-rays in the MV range.

Markus