I though that perhaps the modellers of 1/48 or 1/72 might find a use for this tip:
I picked up some earplugs from work today. You know the type, you roll them in your fingers as you compress them, and they become about 3mm in diameter, then you shove them in your ears and keep a finger poked against it as it expands to its original shape.
I thought these might be ideal for masking a detailed cockpit when it comes down to painting the fuselage. They are ultra-compressible, so any cockpit coaming size would be easily covered, they are cheap (very cheap for me), and useful for keeping your hearing intact if you also use them when you use your power tools. Just squeeze them up, drop them into your cockpit, they expand, but not over the coaming lip.
Just a thought. I haven't tried it yet, but I've seen others using other types of foam for this purpose.