Well as it says ideas on would make me an exhaust with catalyst i could remove later

needs to be 2 1/2 pipework

if you have one like this
http://www.cateran.com.au/cateran/images/technical.jpg
you can make a pipe the same length, with those flanges on the ends. done

i was thinking one more in the can then take the can apart 3 rivets? and remove the catalyst