two screw mount
If you are putting one servo on each flap and each aileron, now you're looking at a 4 oz. weight saving, which begins to be very considerable. Mount the servo so the long dimension runs front to back, use the Hitec hardware and 1/4" birch ply to mount the servo to and you will have no problem.
Most sport planes, even .60 powered, fly just fine with one standard servo driving two ailerons, 44 oz.in. torque divided between two control surfaces. The HS85BB will give you nearly double that torque on each surface, so torque will not be a problem.