The easy fix at keeping the crows from eating your feed during the off season is to set your feeder to go off well before daylight and well after sunset.
I've had good success keeping the coons out by using round-shaped, protective cages that equal the diameter of a 55 gallon drum feeder. It takes a good sized coon to reach the feeder while hanging on a cage that's as wide as the bottom of the barrel.
However, I did watch a large coon one evening while hog hunting that had learned he could reach in just far enough to knock a little feed off the spinner plate. Then he would climb down and eat what he had just knocked off the plate. Let's just say my .270 kept him from making a third trip back up to knock off a little more corn.