Um, I vote an alteration of your "Cobblestones with Mortar" idea. I think it should be five cobblestone, four gravel, but you should get two or three Masonry instead of six. Otherwise you're getting more Masonry than you put in Cobblestone, ya know?
Right, um, and I don't want to rain on anyone's parade or anything, but just a... very unhelpful thought.
What happens if Griefers fill your base with this stuff?
Well that's one patient Griefer, they tend to do the quick kind of greifing, anything that takes 15min to set up for one person generally doesn't fly. This will happen, but maybe once or twice a month, if even.
I don't think people would mind that much, it's a easy harvest if anything.
I think griefers would sooner take buildings down, even IF it would take long.
Also I'm still for my 3x2 natural stone, perhaps 3x3 with a gravel line in between.