Dan,
I would try taping cardboard with smaller openings cut out over the large holes. You can play around with the size and positioning before making something more permanent. If the chickens can just barely get their heads in (but still comfortably) then they cant scoop the food out hunting...
I'm convinced the key to low feed waste is making holes just big enough for the chickens to get their heads into when eating. My version of this feeder is a corrugated pipe with the a milk jug screwed to it.
Cut just enough of the top of the milk jug to fit the corrugated pipe in and screw...
For 5 chickens (or even 6-7) you probably only need 3 holes for them to get to the food. The feeder can be zip tied or hose clamped to the hardware cloth if the main weight of the hopper is supported by the frame. Also the feeder tray can just hang in space if you have it built solidly enough...