There are two places I know of with built jumps on corstorphine hill – the walled garden bit up the top of the hill has no room to build more stuff IIRC. The walled garden bit down on the st johns road side has a bit of room for building and what is there is not much good. However getting to enthusiastic might well upset the rangers. I know thay have removed som innapropriatly built jumps.
Apart from these two areas of built jumps I don't think it is appropriate to build on corstorphine hill. I do know of a piece of wasteland out near newbridge that had jumps built on it a couple of years ago that are now all overgrown – perhaps this might be an appropriate spot and I did find some good looking but overgrown shore stuff out in east lothian. E mail me if you want GRs for them