When you say atv, do you mean quad or side by side? I'm assuming quad. I like my kolpin gunboots. I used to use one on my kodiak. I have one for my ranger now. I also have a great day over head rack I'm impressed with for my general though.
I've also used the simple u shaped ones that mount to the racks. Made by kolpin too I believe. They held securely and I'd be comfortable putting my 50bmg in em but I didn't like slinging mud all over my rifles.