The Siesta clock knobs are tough...does your Siesta still have both forked shafts protruding from the clock mechanism? If so, you are lucky! These tend to be found snapped off or the forks broken. If you have the original shafts still, I've found a seller that offers knobs that work well. (see item CR-17) I also see they now offer knob shaft repair tips too, so if your are snapped off too maybe these could help too, providing there is enough to attach it to.
Here's the link:
http://www.renovatedradios.com/clock_knobs.html