There are some universal overhead gun racks that many people are using on their SxS's. Also, If you were to add a toolbox to your bed, it's possible you could store your rifle in a padded case inside the toolbox, if you can find one the right size. This would also give you an option to hide it away, and lock it up. Out of sight, out of mind....
I would build a framework of wood or metal that fits the perimeter of the bed and sticks up just above the bed. Then I would screw a padded plastic gun case to this framework. This way the gun is completely protected from the weather and scratches and also can't fall out. You would still be able to put stuff in the bed. I would have the gun case laying flat not vertical so that when you open the case the gun is just laying there. If you want to really splurge you could get a foam lined alum case instead of plastic. When done for the season lift the framework out and store it somewhere.
The only problem I don't like about the cloth bag covered gun cases is, it seems like if you leave it attached to your UTV all of the time, in and out of weather, that it would get mold and mildew all over it eventually? I've seen it with cloth ice chest, and other similar fabric covered items.
I would look hard at the seizmik options mentioned above. I especially like the one that connects on the seat.
That's a cool little deal there. Pricey but very cool. Does it keep the muzzle off the floor?
About the nylon bag type cases getting moldy/mildew. I suppose if you left it outside on a regular basis that it would get nasty. But they are just as easy to remove as the seat carrier. I do like the easy access of the seat carrier though.