I've been flying a 1/4 wave AV-14 Rami commercial band antenna
on the belly of my RV-6 for several years. It is a fine performer with the Byonics RTG tracker. However, it doesn't use the NMO mount. I considered one of the small blade NMO antennas but never got around to trying one.
I started out with a home-made 1/4 wave whip
made from 1/8" stainless welding rod and never had an issue with it flexing in the slipstream.