Somebody once suggested I try doing the density rainbow with a syringe. Yesterday, in the process of taking pictures for the upcoming book, I finally tried it, and it worked great. (I used one of those blue bulb syringes you get to clean baby's ears--guess I should mark it so I don't use it on ears again!)
I had much more control over the pouring process when I used the syringe. My husband also poured a rainbow, and he poured his colored solution into a liquid measuring cup with a lip, which also made the pouring easier. Both rainbows turned out nicely, with very distinct stripes.
So, if you've been put off by the difficulty of the Density Rainbow, it may be time to give it another try!