Isn't the purpose of a Lead Sled to moderate (lessen) felt recoil? If bolted down wouldn't that add the wt. of the bench to the weight of the sled (if your bench has you sitting on it you would also have to add your weight to the equation). A body can do it and it might be interesting to compare a totally locked down rifle to the point of impact when a rifle is shoulder fired.
Just finished an article in Backwoods Home Magazine by Massad Ayoob where he stated that using a solid rest for a light rifle will change the point on impact on a target.
You squeeze the trigger, the hammer strikes (or the firing pin strikes), the powder is ignited, and your projectile starts on its merry way. At this point the harmonics begin. These vibrations are what lead FN (Browning, Winchester)engineers developed their "Boss" system. These vibrations are why you can not run a regular four wheel drive locked in on dry roads, or you will destroy your transfer case. Anyhow -change the harmonics and change the point of impact.
Are you simply trying to develop a load, or sight in your rifle for the field? So have fun!