https://www.lbcarco.com/oil-cooler-and-radiator-shrouds.htm
An alternative way to help reduce over cooling – not sure how effective it is. I am sure the MG engineers would not have added the cooler in the first place if the cost didn’t justify the results.
I reckon looking at what’s involved to add the cover to the cooler that most diy engineers would devise their own version of the above.
MGB tips mods and maintenance 