How about
https://www.ebay.co.uk/itm/Triumph-S...8AAOSw2QJa2LKP
Very flat so should give you plenty of clearance. I have a Triumph Trophy 1200 which uses the same 4 carbs, if they run a 1200 bike engine they should with a little adjustment suit the Herald engine. Mine has K&N pod air filters which does look very nice.
I like the idea of the fuel cap but in my mind it would need to be central for it to work. I do like the smooth flowing lines of the bonnet as it is.
Or Weber carb, bit pricey but I'm sure you could source a second hand one.
https://www.ebay.co.uk/itm/MG-MIDGET...QAAOSwrx5UVm5A