See if you can find a prop nut similar to what they use on a 4 stroke for a locking nut. The nut extends down through the prop washer. MOST 4 strokes and some larger 2 strokes (OS 160 FX ) have them.

You will need to enlarge the hole in the prop washer and possibly inlet the front of the prop slightly to accomodate the extended part of the nut. As long as you can get AT LEAST 3 full threads into the nut you will be OK.