OK, now I know what you're talking about. They use it to make outdoor signs
on. It is clear and and hollow ribbed in the center.
Should do the trick, thanks for the idea.

Entity sleepingmonkey1 spoke thus:
> the stuff I am talking about is a plastic and is not compressible.
>> Entity sleepingmonkey1 spoke thus:
>>
>>> use this stuff called sintra or polystyrene.
>> I know what you are talking about, backing board. Its too compressible.
>> Haven't heard any good ideas so far, but maybe browsing Home Depot is a
> good
>> idea.
>>  I was hoping for some material that is like plastic with a lot of ribs in
>> it so it's not compressible.