Diameter of the trim ring must be suitable match for the wheel diameter. Depth of the ring should be very close to matching depth of the outer exposed part of the wheel rim so the trim ring comes close to touching the inner wheel disc. As such, there will be a variety of part numbers for different depth of reach to suit various wheel widths and offsets. Also notice on these aftermarket parts (above) that the spring fingers extend from the outer edge of the trim ring. The outer ring will cover wheel weights, and wheel weights will hold the trim ring a little away from the rim making for a "loose" appearance. To my eye these parts look a little "clunky" on the MGA which is otherwise noted for slim light appearance with rounded corners.
Below is a closer image of an original factory optional Rimbellisher. Notice the outer edge is kicked upward a bit, and that it will be doubled over for stiffness and to provide a smooth edge. This closely follows the outer wheel rim but does not cover the outer flanged edge of the rim (where wheel weights mount), and it will be somewhat narrower than the aftermarket part shown above. Notice also that the spring fingers extend from the inner edge of the ring, which is part of what allows the outer flange to be narrower. This design gives a more sleek or slim appearance on the wheel when installed.
The factory style Rimbellisher was available for the 1500 model cars, but was discontinued after February 1959, about the same time as introduction of the new style steel disc wheels. Slightly different style wheel trim rings were always available from dealers as aftermarket accessories. Therefore almost any style of wheel trim ring may be considered to be a valid period accessory.