Those spider boards look cool! It slightly bothers me that they always seem to be single ended, but I suppose if you want a coax connection you really need to step up to a probe station or solder-on micro-coax anyway.
If you want a cheap probe holder, search for "dial indicator holder" on aliexpress, there are a bunch for $10. They usually have a magnetic base because they are mostly used by machinists who need to hold dial indicators at a fixed position relative to a metal base plate, but they work quite well for oscilloscope probes too.