only reason to stick with one of the standard prop screw patterns is borrowing props from other modellers. for your engine size that would mean the 29mm xM5 pattern but that sounds like it won't work with the bore in your hub. larger pattern with M6 screws will be far more robust and less likely to shear screws...but then that might be a good feature to have in a ground strike, easier to replace a prop and screws that to straighten the crank.

every time i look at my 066 and imagine it on a plane i leave the stock carb boot in place and make up some kind of bracket from the firewall or engine mount to hold the carb.

for what you're doing a piece of 10mm G10 would work for the insulator. another material is that phenolic panel stuff often used as dividers in public restrooms....i imagine that might be easy to find at a recycling/junk yard.