A really simple little thing here, guys. I grabbed some 2mm thick plasticard to use as the basis of the nebulae. I cut it into appropriately wavy-edged shapes, and undercoated it grey. Then I hit it with a coat of gloss black, spattered some drips of white over it and then dusted red or green spraypaint over the rest in order to simulate the appearance of a nebula. I just touched up the edges where the black paint hadn't taken, and Bob's your uncle! USS Lexington shows the size of these pieces.