Without seeing your feeder, we are somewhat at a loss of how to stop the foraging muskrat. If you are using a Blitz dog feeder, the top can be closed by using a piano hinge as we use these to keep the top closed and intact during heavy wind and rain storms. The hinge works a lot better than tape as tape can deteriorate.

As far as the bottom of the feeder being accessed, we were also seeing ducks and other wildlife gaining access to the feeders. We raised the feeders up the pole so that only the swans could reach the feeder with their long necks. The raising of the feeder may help but you will have to be diligent in times of drought so that the swans can still access the feeder with limited water height.

It has been suggested to use tabasco sauce, black pepper etc., on docks to ward off otters. You might try these materials on the pole for use against the muskrat. You just have to ensure that the swans or no other wildlife can rub up against the pole in order to get the material in their eyes as this will cause an irritation. The Regal Swan