I checked around a few Slot Car sites and found this link that appears to be current. Sonic Fiddlestick . The Type D Durometer also looks like a good option for a reasonable price.For rigidity testing, I think a Sonic Fiddlestick is probably the right tool for the job. Unfortunately, it looks like they don't make them anymore (although they're still on the website). Apparently they fetch a pretty high price on that auction website. I'll keep looking around to see if I can find a reasonably priced one, or I'll buy a Type D Durometer.
I can attach my Fiddlestick to one of my adjustable bases and use the Interapid test indicator to detect movement of the blade as it has a very fine tip .4 mm dia.
I might need to get a bigger surface plate.