battleship gun covers

They're often called "blast bags", as well.

 I used Elmer's (white) Glue, in repeated applications for my seemingly never-to-be-completed build of Combrig's Izmail.

I applied a blob of glue, then using a bit of stretched sprue, I'd sweep excess off of the turret, keeping the blob the same width as the gun. After a day or so, I'd repeat the process until I got about where I wanted to go with it.

Tom L beat me to it.  Blast Bags and Blast covers are what I remember from reading contemporary publications.

"Bloomers" probably reflects how they appear after you light off a naval rifle of that sort of dimension.  The purpose of the covers was to keep inclement conditions from getting into the turret spaces.  There's a lot of empty space, if convoluted, arounf the guns, and a number of levels and places that seawater would not be welcome, after collecting a bit.