i made a 12v test light years ago and i still use it. i used a dome light bulb. it's shaped like a big glass fuse, tubular with a conductor at each end. i soldered wires to each end, then crimped on alligator clips.
in a pinch, i have used a multimeter set to read dc voltage and watched for the needle to move for the timing. it's harder to see than a bright light, but it got it done accurately.
and yes, i will second that holding the advancer open works very well, and is accurate. i have used this method and rechecked with a strobe light and found it to be correct.
my only other advice on the dyna would be that the wire tap they supply for the 12v power is not too reliable. it will work if you are careful, but ive had them come open and loose connection is various applications before. i dont trust them. if you have an easy way to splice it in better i would do it. if you use the tap, you may want to zip tie it shut or at least put electrical tape around it so it dosent open up. that has worked out well for me before.
