Here is the solution to mounting a bayonet on an AR-15 without a quad rail or appropriately placed sling swivel stud and without resorting to a barrel clamp style.
This extendable bipod from NcSTAR quickly and easily mounts to the bayonet lug on most AR-15/M4/M16 variants. A push button on each spring-loaded leg can lock into any of the notches to extend or collapse the legs independently to the desired height. The bipod head has a ball swivel joint allowing the shooter to make small sight adjustments without picking the bipod up off the ground.