Damping material for acrylic WG

I'm not sure if this is the right forum for this, but I couldn't find a more appropriate one, so here goes:

I've got some acrylic waveguides for my C12 tweeters, and I'd like to use some kind of damping material on the back side of the waveguide. Any material scientist types that could point me in the right direction, or a forum that might have some expertise in this area? I have some concerns about adhesion to acrylic. And of course I'm thinking of something heavy duty like some kind of clay like substance. Pictures of the waveguide can be found here: Accuton C25-6-012 in a waveguide - Page 2

Thanks in advance for any pointers.

Cheers