As I recall, the cat housing tapered slightly as you pushed the cat down into it. It was a bit tough to get it all the way down without dislodging the interam gasket.
Wipe the metal "can" that is around the cat so that tape will stick well. Use masking tape, tape one end of the gasket to the cat. Use two pieces of tape, side by side, to hold the gasket in place better. Cut another two pieces of masking tape for the other end, and stick them to something close by, so that you can grab them with one hand after you wrap the gasket around all four sides of the cat. Also have a scissors close by.
So, tape one end of the gasket in place. Place a thumb on the taped end of the gasket, until you get around the first corner of the cat. Pull the gasket around the cat, keeping tension on the gasket as you go...you want the gasket as tight and as flat as possible. Don't overdo the tension though, or you will tear the gasket or pull the tape off.
When you get all the way around the cat, use your other thumb again to hold the free end of the gasket while you grab the scissors, and cut the free end of the gasket so that it butts up to the taped end, but doesn't overlap. Then put the two pieces of tape on the free end and tape it down on top of the other tape. The tape will burn away with the first fire.
Now, work the gasketed cat down into the cat housing. Take your time, as it gets tighter, the further down you get.
Hey, I just thought of something you can try. Don't put the gasket all the way to the bottom of the cat, then tape the bottom edge of the gasket so that it will more easily slide into the housing, without the edge of the gasket catching. If you start above the bottom, you'll have to cut off the extra on the top, as you can see in the attached pic.
If you click on this pic, you can see a half-width strip of interam gasket that I cut off of the roll for my SIL's Dutchwest..
