I have recently got two of the new 'top' feeders and have painted/sealed the inside of the 'tanks' with water-based masonry paint.
If I'd thought about it and had had any sand available, I'd have 'salted' the bee-beach area for extra grip. When I eventually use the things, I'll report back if its something I should have done and decide to do with an extra coat of paint.
I really don't like the nuc's integral side feeder, but hadn't thought of using expanding foam filler. However, now that particular seed has been planted ...
I think the only time I've encountered a QX on the entrance slot
to a Miller or Ashforth feeder is on the 'extra storey' Apidea feeder. I'm guessing that would simply be extra insurance against the loss of a valuable Q in a sticky mess of fondant - as per the Apidea's standard downstairs feeder and QX.
Shouldn't be necessary, but no harm adding one I suppose (though remember beespace), if a bit OCD worried. Perhaps the easiest way of adding one would be to use Paynes full-nuc QX flimsy, and the bees would only pass through the slot area. Might be a bit of a prop-magnet though ...