I think I'll chisel the holes into one of my ring layers, then put another (slightly thinner) layer over it; then use plywood for the top wide ring. That way I'll have a comfortable mean to make the holes and also make the whole thing stronger.
How about the wide rim on the posted red ring, how does the owner put the roof tarp over it? Does he just have center hole large enough to pass the whole ring through, then tightens the tarp with some sort of drawstring or something? I'd love to follow this idea, but this is a tough point.