If you have a tensile testing machine, then buying a fixture is the least expensive way to begin radial force testing. To learn about the choice of a complete system vs. a fixture, see the article Knowledge Base: Radial Force Testing – Complete System vs. Fixture.
The diameter of the opening may be varied by positioning the arm of the compression station. There is a known relationship between the arm position and the diameter of the opening, and between the force on the arm and radial force applied by the object in the station. See Knowledge Base: Measuring Radial Force. The tensile tester machine controls the position of its head and measures the force applied to the head. With the proper equations, this is equivalent to controlling the diameter of the opening and measuring the radial force applied to the compression dies by the object being tested.