The avian vet I take the hens to showed me a trick.I get them to stand on a scale too, though without the mixing bowl. It's a bit haphazard, usually takes quite a lot of mealworms, time and patience to get both feet on, and in the right direction that I can still read the display, but when needs must, this works for us.
Put a small, rectangular plastic container with straight sides on the scale. Put the hens feet in the plastic container. She'll usually stand still. It's something about the feet being confined as on a nest. I did this all through last summer when I was crop feeding Lorna. She only jumped out once or twice
I doubt it'd work for roosters, but it might be worth a try.