I've made and engraved quite a few MotoX bike clutch casings. If it's just an engraving then Jonathans way is the best and easy enough to do. Depending on how curved dictates to how dense you need to probe and time taken, thou it's usually a 5min affair has your only needing the curvature not the whole shape etc and the surface area is small.

Also I have a USBCNC card to try but not got round to it yet has the machine I'd like to try it on isn't finished. Will let you know when I do but it won't be for awhile.