I have template for just this. It's hand drawn by the previous systemdek owner. I haven't made it, but should be a good start for you. Send me an email I'll send it back as a scanned pdf attachment.
Where to get an arm board?
I finally received my RB300, looks sweet! But, wait just a second, of course it does not fit into my tear shaped arm board in my Systemdek iix. I searched, but really found nothing. So, where does one get one? I'm not all that good with wood, so I'm not sure I can successfully make one. Any ideas would be welcome.