But I'll probably go a little wider to 3/4", thanks. That plus a 3/8" rim on the gutter profile, takes 1 1/8" out of the open width(2 1/4" off the opening length). I don't see much gained by going any wider than that. Deeper helps.

These will not be sloped. That's one of the great things about fiberglass, the gutters can be more effective. Wood is not as adaptable. Some of the boats built in the harbor use U channel bronze, even some have plastic channel. There's a zillion ways to do it.

I think Westy asked this: I show a rounded bottom in the channel but it may be square with some thickened epoxy and rounded stick forming a rounded bottom.