Kneel on the board on a cement floor. Then stretch it in front of you. Your weight is the solid structure.See, that's the exact feedback I was needing, thanks! It's approximately 3' x 6' and I'll either do one frame with a couple supports or two roughly 3' square ones.
If I were doing it on something sideways, like a run, I'd sandwich the end of it between a couple boards and have someone hold it tight, kind of like a come-along when doing wire livestock fence. But since I'm making frames and not pulling against a solid structure, it's a bit different.