Don't worry Cane, your custom catchables don't have to be fish! They can be any object or even clothing if you wanted. You're right, don't put the fish script in it -- just build it however you like.
Then add it to your local catchable prize server in the same way that you would for anything else. To make it SUR, use rarity mode and be sure to add it to the SUR notecard (See the CPG manual).
Also you might like this thread on setting rarity for the prize givers:
viewtopic.php?f=6&t=63P.S.: If you have the same name in SL, send me an IM inside of SL and I'll add you to the verified users group here!