My monster mud horse is stored outside and covered with black plastic....works just fine. My other horses are still in the garage because they are not monster mudded.We've met this problem by factoring in something we cal 111 Compliance. 111 means November 1st, and what we can do with a prop on that day. We have to take 111 Compliance into every design we come up with, and includes how a prop can be disassembled and stored, what the effects of long-term storage will be, do they need to be stored in a climate-controlled space or not, etc. This enables us to always make props that can be broken down and stored instead of ones that eat up a lot of space.
If we can't make a prop 111 compliant, it doesn't get made. For example, I've been wanting a Headless Horseman for years, but to me a horseman with no horse just doesn't work, and I've yet to come up with a plan to make a 111-compliant horse that looks real. Someday I will.