I use the "U-shaped" rod external rudder lock on my -7A. It is adequate and cheap. Two issues to consider:
1) On a couple of occasions when the plane's been tied down outside, the rudder lock has withstood a 40 - 50 knot wind directly from the side, but the U-shaped rod was bent enough to make it difficult to remove. The lock was on the "downwind "side from the wind and had "worked" up in the holes. If the lock was on the upwind side, it probably wouldn't have been an issue. I haven't figured out how to keep the lock from "working" up. After "working" up and down in the holes during the last 4 years, the holes are starting to get elongated. It's getting to be time to drill holes to mount one on the opposite side. Bottom line: the rudder lock took the abuse, not the rudder. It's easier to make another rudder lock.
2) Even with a "Remove before flight" flag, I've strapped myself in, started up and found I couldn't taxi, so, of course, I have to shut down and get out to remove it. Also, since its external, the rudder is vulnerable on real windy days unless you have a right seater to put their feet on the pedals or make sure the plane is turned into the wind by hand before you remove the lock.
Frankly, I didn't know there was a an available internal lock. I vote for that, because my internal control lock on the stick is very handy.