There are certainly subtle variations under the rear deck. The plastic tray fits because its plastic and shallow. Making these in bulk in metal is very risky if each one has to be worked to fit, and contacts anything around the axle, especially the brake pipe. This must be why nobody has taken the plunge, because it would be a good seller.
Morgan make a lower cowl Jay, but the basic cowl is just a pressed metal box to deflect air into the lower radiator. For a while they advertised the 'lower cowl' which extends the standard cowl below the number plate box or full bumper, but this was no longer listed last time I looked.
Then there's the Aero Racing front splitter with two circular holes - at a price. But for Bumble I just wanted a simple cover, and they don't offer one.
