I removed the silly aluminum angle Livingston used to cover the transom.
I used a heat lamp after to ensure the plywood core was dried. Then I lightly ground the transom and laid one layer of Nytex and polyester resin to close the gap and seal it permanently.
Once done I clamped my two pieces of 1/8" thick aluminum, one on each side of the transom evening and squaring to each other and the transom. Clamped them hard into place and drilled two 1/4" through holes for bolts.
Then those holes, I enlarged and filled with thickened epoxy.
The two are enough for my engine.
I used some matching gelcoat , two colours, as the interior of mine is grey and the exterior is white, to blend in the Nytex. The colour split is covered by the riser.
At that point I cleaned up the alum. edges and corners and bolted them into place along with some caulk.
I could now measure fairly accurately the gap between the two plates and cut a piece of UHMW poly to fit between. I cut that so there was a 1/8 top hat each side covering the alum. plate edges so that the engine clamps didn't scrape their way down, peeling paint. Some small bolts were used to secure the filler.
I used the natural UHMW as I had some. Were I buying I would get black for the UV protection. The white will break down although I expect at least 10 yrs.
You will need to experiment to find the centre point and mark it with a pencil on the alum. as the little keel and the engine drive housing must be in line or there will still be a very small rooster tail, but no big blast of water to come back into the boat. I circled the clamp pads with the pencil once I was happy with the final placement. The graphite pencil will become permanent on the aluminum.
Unfortunately I don't remember that final height and I'm no where near home so I cannot measure or take photos. The engine was 14" from the top of the clamp ID to the anti cavitation plate so the riser is around 13 - 13.5" to the bottom of the small keel.
It made a huge difference. The boat is faster by about 5-6 mph with just me, at about 19mph, as measured with a handheld GPS [friends] and it will now plane, about 11-12 mph with both my wife and I in the dinghy. Before it would not plane with both my wife and I, only me.
My dinghy is the 9' Livingston with an 8HP Yamaha, two cycle, short shaft.
Over some time before I had used the alum. pieces and an ugly piece of wood for a practice riser clamping the thing into place with two cheap C-clamps. I cut small blocks in 1/4" increments as lifters to determine the final height and CAREFULLY kept raising it untill I got some aeration. At that point I dropped it about 1/2" to ensure the prop remained buried but not to deep.
Nevertheless, observing wildlife from the trawler works, and I prefer to acquire/consume sea creatures from a supermarket or a restaurant. 
