I'm sure hardcore custom content creators will have more advice for you, but here's a tidbit or two.
From what I hear, it's good to set the SUR catch rate very high. If you think about the fact that the people are only catching a SUR 2% of the time to start with, the default 40% is very low. It means they only get one of your customs 40% of 2%, or less than 1%, of all successful catches... And if you consider the catch rate for a Level 5 person is 65%, that means they'd catch your SUR maybe once in every 200 casts (if my math is right...)
If you have more than one SUR, then it becomes extremely difficult for people to collect them all, particularly since they aren't targetable. Some people work around this problem by not only upping the catch rate for SURs, but also separating their SURs into multiple prize givers so you get a shot at each giver every time you catch a SUR. (Thank you for that tip, Amara!)
Of course, if you WANT it to be that insane-o difficult, then that's your perogative of course... I've just heard of a lot of frustrated completists out there!
If it helps, consider your percentages in light of the person's typical chances of catching an item, which you can find here:
http://7seasfishing.com/stats.htmlFor a level 5 person, that's...
Catching: 65%
Of successful catches, it breaks down as follows:
SuperUltraRare: 2%
Ultrarare: 9%
Rare: 19%
Uncommon: 45%
Common: 25%