Here is what the mostly finished frog looks like. It will be screwed to a mortise in the infill, which will be drilled so a screw driver can reach the screws. Believe it or not, the screws are accessible (with the blade removed but with the cap in place). Real close on the center screw.
If you have looked at the mouth block on traditional infills, it is fitted as an extension of the sole, as in this example (image from Peter McBride’s excellent article “making a dovetailed bench rebate infill plane“). Using a movable frog negates the possibility of the iron seating on the sole, but this is not an issue in my case. The back of the 1/4” iron, sharpened at 30 degrees, is above the 3/16” sole so it would never touch. You can see this in the photo above (a bit exaggerated).