No need for a radar gun... a handheld gps that captures max speed (like a garmin etrex) works great. We use it on the aluminum boats and ski boats alike. The 15hp's we have of various vintage all run 24mph consistently, and have touched 25-26mph briefly under ideal conditions. With a light load, they really haul- enough for my 150lb buddy to ski behind!
I checked and dont have any good pictures of my mini jacker installed... here is the best that I have. I mounted mine as high as I could on the transom, but could not go quite as high as I needed due to the lip of the aluminum at the top (hence the additional board under my bracket)... but its high enough to get a nice solid surface to clamp against.