You can use 4 in (bout 18 inches long drill little holes on the bottom to let water through if it rains) PVC with a Y at the bottom and a removable cap and zip tie the body to the door.Like those deer feeders.
This is what I use, Mine are 4 foot tall and I only have to fill them 3 times a month. The dogs seem to eat less and look a heck of alot better.