The 1907 or MRT sling is the way to go.
Here is some good instructions for its use.
Maybe I miss read you but you can't use the sling in offhand.
Make sure you get a larger coat then you need, the way to dampen the pulse is with heavy sweat shirts under the coat.
After you figure out how to put on the sling, practice, mark the sling ( a ball point pen will work) so you know where to put it for each position.
Before the match, spend as much time as you can in the sling dryfiring.
When shooting HP slow fire at 600 or 1000 yards you normally shoot 20 round strings. After 10 rounds, (or sooner if you need to) completely come out of position and re-adjust the sling.
I highly recommend you get the AMU Service Rifle Guide from the CMP Bookstore, best $6.95 you'll spend for rifle shooting.