Another solution would be to use 138mm round filters intended for Motion Picture / Broadcast applications. This is very common size used in matte boxes by ARRI, Chrosziel, and others. Schneider, Tiffen, and Formatt all make polarizers in this size.
If 138mm is still too small, you could look into the various rectangular filter sizes also available for Motion Picture / Broadcast applications. These include 4"x4", 4"x5.65" (Panavision size), 5"x5", 6.6"x6.6", and several others. Polarizers exist from Schneider, Tiffen, and Formatt in these rectangular sizes as well.
The only problem with all of these filters is they are designed to be used in filter and matte box systems which incorporate all of the necessary mechanics to hold and manipulate the filters in front of the lens. Outside of a matte box, there is not an easy way to hold the filters in place, without developing a custom filter holder.
I have seen several companies, who cater to the motion picture industry who develop custom lens adapters, filter holders, and other random gadgetry.
Paul Duclos of Duclos Lenses is well respected in the motion picture community. see www.ducloslenses.com
Lentequip is also well known for various lens / filter / matte box related accessories. see www.lentequip.com
I will have to refer to some notes to find more contacts in the motion picture industry for this type of application.
You might be able to use components from ARRI or Chrosziel light-weight matte box or sunshade, to accomplish this as well.