I had the same problem, I found my breeding rats always upturned their food bowls and made nests out of them.
I got a length of pvc tubing, cut it on a 45degree angle, then drilled holes near the edge, and put thick wire through the holes so it created bars across the opening of the tube. It's a gravity fed type thing, I just fill it up from the top of the tube and they eat it from the bars at the bottom. I just drilled two holes at the top of it and made a wire loop to hook it over the edge of the tubs and the lid holds it firm in place.
Only ever had problems with one of my rats climbing in the top and sleeping in the tube, but that was the prototype and I've made the rest so that they sit almost flush with the lid of the tubs. I haven't had any problems with the rats chewing on the pvc tubes so far.