I've mentioned this before,but for maybe for some newbies it will help out.
To mask turret openings especially win an interior is done I use makeup applicators.Very cheap at a dollar store and can be cut to size.For small openings just cut slightly larger and push into the opening.
Hope this helps some guys.

Tom

Thanks for the tip, I usually use liquid mask.
DJ

I also use it for the smaller open hatches and cupolas

I will have to pick some up. Thankfully, I am in the land of "dollar" stores.

Even cheaper - roll a newspaper sheet and stick it to the turret hole.

The sponges are reusable. This is just my opinion.