I've been thinking of ways to change the design from the SO site to eliminate issues with the fins.
I think replacing the metal box with a piece of sheet metal on the bottom would work. Then, two eye bolts could be screwed into the plywood (or whatever) to hold the clamp down. Therefore, there is no interference from the sides of the box.
I also found this one.
http://water-rockets.com/article.pl?117 ... pFlwHA9VhA
I really like it. It allows for you to fill the rocket after it has been attached, which gives you a lot more accuracy when filling the rocket, and keeps you dry. Unfortunately, I won't have a spigot anywhere near where I'm going to launch it, but I could probably rig something up to just drain from a 1 gallon jug or something.
Plus, unlike a lot of other launcher's I've found on the internet, it only uses common parts that I could find at a local hardware store. Sure, it takes a bit of plumbing, but it will be worth it.
The only issue is that you must cut off the little ring from the cap. But I don't think that's going to present any issue with another launcher either, so it should be fine for competition.