Ok, let me clarify...  The aperture is just over 0.1 dia.  The metal at the top of the plate is extremely thin.  It's like looking at the edge of a sheet of paper.  Imagine if you will, rolling a piece of paper into an aperture about 0.1 dia., and then looking at the sight post through it.  That's how the top of the aperture looks as I'm sighting with it.  This may come back to bite me in the posterior... but I'm going to give it a go next weekend at Kalamazoo.  If it doesn't work, then I'll come up with a new plan before the Roosevelt.