No one currently repros the plenum in cardboard. You would have to source a viable original one for that. That is the best way. Good ones can be 150-300.00. with time and effort a damaged one can be paper Mache repaired back into the running.
Thanks Bob. After I took mine out and noticed it was beat up I did a quick search and discovered it didn't appear one of available. I put mine back in the car. The good news is almost all the broken or previously repaired sections of the cardboard are out of sight. Being under the dash helps.
I'll stay on the lookout for an original that could be repaired and still open to someone who has one they can pay with. The part number is C4ZA-18471-C. Does that mean the plenum fits in many year cars or does the ending C mean there are changes just for the 68?