For things like pluging up landing gear bays and engine intakes/cowlings on airplanes, I like to wet down toilet paper or paper towles and shove the in the opening. When it is wet, you can get it to fit in just about any opening, and when it dries out, it is stiffens up. When you are done painting, it comes out pretty easy. Or you can put a couple drops of water to soften it up if it is stuck in tight.
I’ll often use foam pads used to apply makeup. A whole bag is very cheap, and they expand to fill holes quite easily - even large areas like landing gear bays.
I usually use Kleenex, but if the opening is simple enough- say a rectangle, I will cut a piece of cardboard and fasten it in with blue tack, from the back. I can push it into interior and fish it out with tweezers.
I use expandable ear plugs if the holes to mask are small enough. Just mash em between your fingers and shove them into the space. Then they expand to conform to any shape. Eazy in,Eazy out. I also keep a pair handy when my SWMBO complains about how much I am spending on my hobby.
For open hatches on armor,I use cut up make-up sponges.For landing gear bays,I use dampened facial tissue,for cockpits,I use the canopy itself or stuff it with dampened facial tissue.