I would suggest a trough feeder from Wynola ranch website. You can place wire mesh over it. I use 1'' x 1/2'' vinyl coated wire. But you can really do anything at that point to create a barrier. since it attaches to the front of the cage they have to stick their heads out far enough and when...
hows it going? I did something very similar a while back except i just used a hole drill and put a 1inch hole in the top and set a funnel in it.
Google low waste feeders. I make them out of 2 litres all the time