I modeled this after the design of a wheeler engineering scope mount I saw for sale at a local shop. The clamp is placed on your barrel and another small level is placed on the mounting base of your scope. Alight the barrel clamp so the bubble matches the level bubble placed on the mounting base.

Then place your scope in the mount and place the bubble level that was on the mount on the turret cap for elevation adjustment. Turn the scope so it matches the level in the barrel clamp and your scope will be mounted parallel to the horizontal and vertical axis.

I took apart a standard line level purchased for $1.47 at one of the big box stores to use as a bubble in the clamp. It also uses 1/4x20 bolts and nuts which are molded into the lower clamp.

I printed this without supports or a raft without any issues.