Nice job there Wheels. Cheap, functional ingenuity. Maybe you should make a pattern and sell them on ebay for people that like to buy things.
I don't like picking hardened clay out of the fins either so I made a flap similar to yours. I used some 1/8" mat that I found in a dumpster at work. I'm not sure what it is but it's firm enough to not flip up into the tire but flexible enough to bend when I go over a log. Anyway, here's a couple of pics.
First I made a 'high tech' pattern to get some perspective. When I was comfortable with a shape I cut the material.
Installed. It might be a tad long but it can be trimmed if it becomes a problem.
Also, I followed the lead of another member in this post
and made a mud guard for the front of the swing arm. Lots of mud & crap collects in there too. I made this one out of 1/8" ABS which is very rigid. I formed it to fit with a heat gun.