Try using a thin nut, as suggested, then your aluminum spinner nut- the aluminum nut doesn't have to be too overly tight.

Also, if you are using a WOOD PROP then after just a little while no matter how tight you make it, the wood compresses, and the nut/locknut combo will be loose.

The answer is before each day of flying, and maybe once during the day, tighten the flat nut, then the spinner, or locknut.

As mentioned, try not to run the bottom end adjustment too lean.