If someone wants to make a nice closed-airlock-door block texture, as a 32x32 .png, and post it in this thread or send it to me some other way, I will include it in the mod - will take me less than 5 minutes to update the code, it's an easy fix.
I think it should be a dark-ish grey colour to match the airlock frame but not as dark as that. I don't like the yellow/block cross hatching (Ez's screenshot) for the airlock, I would want it one colour only.
If anyone wants to get fancy, it can maybe have some kind of diagonal or step-wise lines on it (colour matched), as long as the blocks tessellate OK bearing in mind the airlock frame might be 3x3 or larger.
My own concept of the airlock is it works with sliding plates like a camera shutter:
It's possible I will code something some time which draws it like that instead of using blocks at all - I would find that fun to do and I'm getting better at making in-game models (see: Display Screens) but absolutely no promise that I will find time to get round to this, I have approximately 100 things on my GC to-do list.