I don't know if anyone makes them, never looked, but really it's not much more than some flat metal. I think I remember you either had access to a welder, or knew someone who welded so they could put one on pretty easy.
To figure out what general angle to put it tho, I would mark a spot below the handle on the boat floor and try and take a tape measure with you out on the water and wind her up. When you are cruising and have the prop just slightly below the surface of the water, measure the distance from the handle to that spot on the floor. Then when you are on shore you can hold the motor in place to setup the proper angle. That should get you in the general area, but I'm sure it'll take some fine tuning once attached.
Being on a little four rivers, if someone does make a cav plate for that motor I'm not sure the angle would be right unless they make one that is specifically for long tails that are mounted only inches out of the water and not on the top of your boat transom.
(I should say that none of my theories with this have been proven. I was going to do these things with that mud motor I had without a cav plate but never got around to doing it before I sold it. Interject your own logic accordingly)