The
Gun Shell on those QF 4" Mk XVI mounts was really just a '
rain keeper outer' and not really capable of offering any great protection to machine or man
We have an image from HMAS Swan [same gun mount] in New Guinea [Tropics] where the top of the
Shell had been removed [to get better ventilation?] to disperse gasses during operation [and a few lengths of angle welded to the shell for a canvas roof to keep the tropical rain out
The maximum elevation for this mounting was 80 degrees, so you will need a sizable canvas bag around each barrel to fill in the large gap you have made
Derek