Instead of rabbeting, you could simply drill out sections/recesses for
the flanges with an appropriately sized Forstner bit. Even if the
flanges are much longer than wide, you could drill side by side holes
down to 3/8", leaving 3/8", and clean up the resulting recess with a
knife or chisel.

That's an excellent idea. Much better than resetting the router
table.
I'm going to try it over the next week.

Thanks for that tip.

MJ