I would not bother using a lead sled, you will risk breaking your stock and it won't tell you a damned thing! A double needs to be shot off the shoulder with the front hand controlling the rise and helping with the recoil. This can be easily accomplished by draping a 3/4 filled shot bag over your shoulder to help slow down the recoil, or better yet, go and buy a strap-on Past recoil pad. You only use it when you need it. Then support the front hand and arm but putting it over a padded front rest of some sort, but keep your hand in control of the front end of the gun. You can shoot it safely and comfortably this way without fear of being slammed too much by recoil. Just be sure you have the front rest as high as possible so that you are not leaning over and into the rifle too much. Shot this way, the gun can still freely recoil and it won't throw off the shots. Your body can still flex from the waist up.
If you don't like this method, then you need to build a standing rest. All of the rules still apply, you need to rest your hand and forearm on a padded rest, but your off hand needs to hold the front of the gun in a naturally recoiling hold. This will duplicate an offhand shooting hold for you and you can still use a pad of some sort. Bob