Rustydog,
Here's what I'm going to do. I got the idea from another guy that I went out calling with from here awhile back. Get yourself a Gunboot. Then, cut the notch to fit the lower of the AR. Next, find yourself a large, heavy duty fleece neck gator. Make sure the neck gator has a pullcord on one end with a toggle. Get out your rivet gun and drill. You're going to pop rivet that neck gator to the lower section of the gun boot. You obviously need to make it go beyond the notch so that dust/snow will stay out. After you have it all riveted together, you can easily slip your AR in it and pull the cord on the gator tight. It works really good and kept all of the spindrift snow off the gun. It stayed put too and worked like a champ. With that lower riding in the notch, it really stays put. Those directions are fairly crude, but you should get the idea.