blueygh2: Yeah, that's what I was thinking, but I definitely don't have any foam tubes (or similar) and I'd be worried that the pins would change the shape. Also, the circumference around the upper arm is much larger than around the lower arm. I wonder if I should just steal a mannequin's arm from a shop window or something! :P
Ingrid: Yes, that is a possibility.. but I'd still be worried that it would create ridges (like imaginary seams).
Thanks for the replies!