You should check out the mod Procedural Fuel Tanks! Not only does it let you make tanks with the size and shape you want (normal, cones, spheres etc) but even more relevant to your install it lets you pick what texture you want it to be. This way all your rockets can really look special, as you can choose between the many different textures, or import your own. Anyway, might want to give it a look, as it also saves tones of RAM allowing you to use better textures.