The RCM spacer is an aluminum version of Ryu James's CF spacer but one side is a crecent moon shape to make space for the brake disk/calipers.
http://www.rc-monster.com/proddetail...rontmotorcover
Select the .25" brake spacer from the drop down.
The RCM spacer is the best design available (in my opinion) as the spacer mounts to all four motor mounting holes and uses two seperate mount holes (the RCM mount has six holes total) to mount the motor which eliminates any potential winding damage caused by long screws. The only thing I see that would make this a better design would be to add space in the center of the spacer to fit a 5x10 or 5x11mm bearing to increase the rigidity of the motor shaft as the pinion must sit .25'" farther out on the shaft. This additional bearing should eliminate addition stress and wear to the existing motor bearings. You can also drill out a hole to fit a bearing yourself as well. With that said, I have used the RCM spacer for months and have not noticed any ill affects. The Castle NEU motor shaft should be long enough to fit a pinion without the need of a long shank pinion but as for other motors out there, they will most likely need a long shank pinion.