Let's cut to the chase - If you "lost" the fin while you were sailing, you didn't have it screwed in properly.
I've sheared at least three powerbox fins clean off at the base (Two on the reef, one on a sea turtle. Sorry, Mr. Turtle!), and the base was still in the box after the collision. I've sailed hundreds of hours on dozens (and dozens) of Powerbox fins, and never lost one out of the box.
Honestly, you should have someone else show you how to put in fin screws if you're losing them even from tuttle boxes. Especially if you're shimming the fins with fincover material. Not even the worst US/A Box fin would ever need something that thick and spongey.