Humidity will be your worst enemy. The high heat inside of a garage might cause excessive shrinking too. You might get some cracking around the eyes and mouth.Not much you can do if it has to be in the garage. It will be more prone to getting the little moth larva that eat the roots of the hair, making it fall out too. You could probably spray for those.

Do you have a pro shop or sporting goods store in the area that might be interested in displaying it? If it were my mount I would find a way to get him inside a heated and cooled house, or store. Life in a garage will not be kind to your elk!!![&o]