But in Et's design the skeg box doesn't go thru the hull and, I think has a better look on the outside.

If the exposed edge of the box bothers anyone it could be mounted completely inside in a similar manner to Et's. I went for the penetrating system because the fitting is easy. Another option if you don't like the look of the plywood edge showing is make the skeg box out of cedar strips covered with glass. If you use the same wood you stripped the hull with it would blend in pretty well. The visible edge of plywood doesn't bother me, it is an honest part of the boat, nothing to be ashamed of.

One feature that this skeg has that is a bit unusual is that it is offset to one side, meaning that it is not on the centerline of the boat.

This is an idea I stole from someone else. It makes it much less likely to jam and also easier to stow gear in behind the skeg. Note that any skeg including Et's could be offset similarly. It only needs to be about an inch or so. Of course if for some reason you don't want it offset, it doesn't have to be.

The level is being used as a straight edge, and the blue triangle has a lip on the underside of it.

This is a good way to double check any skeg, offset of not.

As for as up and down placement, I going to leave that one for Nick to tell you.

The plans include marks for how deep the skeg box should be to be sure the skeg itself is not sticking out when retracted.