I like the rear mount idea WAY more than the handlebar mounts. To easy to hit branches. In addition, if mounted to the upper frame , I'd imagine the rider weight would soften the impact of bumps and pot holes getting to the rifle, more so than if it were mounted on the forks. If on the forks, I'd be worried about scope holding zero.
I'm going to work on making a rear mount setup for my XL600 that will accept a std gun case, modified to mount on the bike, and lockable.