Mud hasn't really been an issue on most hunting trips. I had a gun boot on my 4-wheeler but normally just used the same forks for the field ride in. I have a couple of beaters that sometimes gun along on rides. That $40 spent was much better than the accessory rack, the boot mounts, and the boots.
I used the PAC RACKS to start with. Had on 4 wheeler and really liked. 3/16 x 2 flat metal heated and bent for a 18" with. Tapered the ends. Welded on all thread nuts painted with Rustoleum bed liner mounted to roll bar mount. Seems to be working fine.