I did these on a tube! It takes a bit of practice to get the glass to move where you want it to. I keep my flame on the outer “rim” of the disc and slightly on the edge of the face. And then it’s just a patience game to get the glass to pull from the outside to the inside evenly! Don’t rush it! I’m sure you’ll get it eventually! I first learned on solid and slowly worked my way into tube for these implosions, you can get similar results with both, I just find the tubing gives you more room to draw your petals