Reprising my previous post, I think most people would suffer less frustration if they have something to hold the tubes in place for tacking. I built a really nice frame using a small machine table as a base, for example. I didn't take pictures of my setup, unfortunately. I wish I hadn't managed to screw up the frame getting a stuck seatpost out.
Other ideas are kitchen counter offcuts or even some MDF. Although wood is expensive nowadays, it doesn't grow on trees, you know (h/t James Krenov)