Here's a simple one you may want to try
Alan.
It's nothing more than a Thumb Screw - available in various lengths and thread sizes at any hardware store - with a hole drilled in it and a knurled nut to lock it in place. It's adjustable for elevation, but not windage. You may need to drill and tap the tang to fit it.
Another option if you're handy with tools. This one is fully adjustable. It can be fabricated with a a little ingenuity using a small threaded machine screw and piece of flat steel, aluminum, or brass bar. Cut, drill, and file the rectangle to your liking. Drill and tap a hole for a machine screw into the center bottom of the rectangle. Install the screw and secure it with Loc-tite, solder, or epoxy - then cut off the screw head. Drill and tap the tang to accept the screw. Purchase the aperture disk on line.
As with the thrumb screw, it's screwed in and out of the tang to adjust elevation and a knurled nut (or threaded disk) screwed down against the tang locks the elevation adjustment in place.
