How to tighten a circular saw without a spindle / arbor lock?
I have used a screwdriver. 

UNPLUG THE SAW.  

finger tighten the bolt as far as you can, set the screwdriver across the fence and slide it into the teeth so that  blade tooth is up against the shaft of the screwdriver and can not turn. This is usually enough to get the arbor bolt to snug up enough so that it pinches the arbor onto the blade. Perhaps a vise grip would work as well to grab the blade.