[riddleofsteel]

The old Remington 40X target rifles had a threaded insert in the forend with a screw and lock nut. The internal end of the screw had a widened nylon insert that rested against the barrel. You could adjust barrel harmonics by tightening the screw to affect grouping. Once you found the proper tension for your load you tightened the lock nut. I always thought it was a good system, especially if you shoot a lot of different loads from the rifle.
It would a simple project to bore a hole in the forend and install a threaded bushing. Most automotive supply or fastener suppliers stock screws/bolts with a nylon insert in the end. Or you can just bore out the end of a bolt that fits your insert and install a nylon or plastic insert.

Of course if you mainly shoot one or two loads as is the case for most hunting rifles, the shim method may be simpler easier and less hassle.