Center brace. See how I rounded off the sharp edges of the stopper using a rotary tool's cuttoff/grinder wheel. This will prevent any damage to the plastic propeller blades when the prop hits the stopper. You can purchase a very good rotary tool (kind like the Dremel) at Harbor's Freight Tools for like $10 and change.
This is the exact one I bought and used.
http://www.harborfreight.com/cpi/cta...emnumber=41695
Cutting out the groove
MJ mod mount
Surface agitation