Amateur tip - Make shims out of used plastic food container lids (I know this may be common knowledge, but I thought I'd post anyway.)
For the Hobie 17(/18 upgraded) rudders "Molly McButter" bottles are nearly the same sizs as the contact area for the upper castings. The lowers are smaller.
I used 2 "I Can't Believe It's Not Butter" lids and made 8 shims.
1) Eat baked potatoes, with all the fixins (especially bacon), and put I Can't Believe It's Not Butter tub lids in the special basket in dish washer (so they dont fly all over and melt onto the heating element).
2) Trace appropriately sized circular object onto lids as many times as you can.
3) Cut shims out with scissors.
4) Punch holes in center for bolt. I used a hole punch and "nibbled" out the hole till it fit.
5) Install on rudders.
6) Look at crew, and say (using a Fabio voice) "I Can't Believe It's Not Rudder...........Shims"
7) Laugh hysterically, then explain the Fabio reference to crew. Youtube is your friend here.
For you skinny, health conscious, people, you can use Crisco lids and make more, but omit steps 6 & 7