I shoot big bore rifles as well. 7mm Ultra, 338 Ultra, and 375 Ultra. I use the lead sled but do things a bit different than others. I went to the local sports store and bought a 100 lbs dumbell. Took a dremel tool and carved a half moon into the handle so it sits on the tray. This seems to work awesome. The gun and sled never move. The dumbell was much less than buying lead shot in bags. I can shoot all day at the range and not feel a thing.
I also I agree with putting a top notch scope on your rifle. I see so many people at the range with big rifles and end up blowing up their scope cause it because they spent all their money on the rifle. Remember, if you can't see what your shooting, your gun is mo good. Keep in mind, a 338 Ultra mag has 96 lbs of recoil. Recoil is totally subjective. I am a 225 guy and recoil is not a bother, my father is bigger than me and can hardly handle a 7mm mag.
Hope this helps....