Option 1
Of the four versions this is the simplest. This mechanism is simple and provides protection against grounding by being heavily sloped and by being at the extreme end of the boat. On grounding the stern will rise as opposed to component breakage. I did not invent this idea, I got inspiration from Jeremy Walker of Southern Outriggers in New Zealand, formerly of South Africa and his 23ft proa. This option is simple and doable.

Option 2
The most refined. The trouble is that it seems very difficult to make. How to build a mechanism that activates the trim tab yet can still deal with the board being rotated by 90 degrees

Option 3
This is quite simple to build. It has the advantage that trim tabs are not required. The rudder can be a simple one piece plywood board that rotates 180 degrees. The rotating left and right is done in conventional kick-up rudder style. The problem is that a V shaped notch needs to be cut under the bow so as to allow the board to turn left and right. The bow transom will need a narrow slot to allow the board to rotate upwards when not in use. The upturned position when not in use is not really the downside it first appears. In some way it helps to balance the cente of effort for the hull by balancing the rear sail bias

Option 4
This is a more refined version of number 3. It stows the board away when not in use. It has the advantage over option 2 in that the board is further aft, giving more precise steering control and better ability to align sail center of effort with CLR. The downside is that it still retains the drag creating notch on the underside. Additionally there may be issues in terms of structural integrity that the large slot creates in the hull

Option 5
This version seems mechanically straightforward to make. It has the advantage of no drag inducing notch. Rudder blades are simple to make one piece plywood (or wood). No trim tabs are required. The roundish shape that meets the water would create less drag than the flat transom of earlier versions. The drum would pivot on a vertical axis and be approx 40 to 50cm in diameter. A slot is made in this drum to accept the pivoting rudder blade. This drum based rudder version seem to be a good fit for the blunt ends of scow-proa

For background information, the leeboard is held in position with light lines. In event of a heavy grounding the lines can break and the leeboard pivot backwards, absorbing energy as opposed to causing structual damage

My present thinking is that Option 5 the drum style rudders are the best way to go. I say this as I feel they are a good fit for the hullshape of ScowProa. This method looks mechanically simple and straightforward. It may not be as refined as some other versions, but seems sounds, simple and effective