The next step is to source a bunch of glass test tubes. I found mine at Paxton Gate
in San Francisco, but small piece by piece orders can also be placed online at American Science Surplus
Once you've found some appropriate glassware, carefully measure each of your test tubes, since in my experience, they are all slightly different diameters, and it's crucial that the supporting panels hold the test tubes snugly.
I got three different sizes of test tubes thinking that it might be nice to hold different size flowers, but I think if I could do it over again I'd get test tubes of all the same size so everything would be interchangeable.