would using nails mean that cracking is more likely?
on the issue of holding an 18kg plaster board above your head with one arm, with upto 9kg dispersed over maybe 1200mm each side of where you're holding it would surely damage it's structualy integrity. minuet fractures and breaks would surely form in the board. I know you cut some off, but often, not much.
have a think about what holding the board one handed is doing to your body. for the sake of £20 for a cheap pair of props, it would seem to me that you aare being a bit nieve and may come to regret it in your 60's when your body can't handle the day to day stuff. sometimes it's better to be a bit of a fanny