This is what I've made up for shooting kitesurfers, well almost:



In reality the front, the bell shaped end, is best to not be there.

Before I expand on that though, your comment about the images not being sharp at full zoom worries me, as all a device like this will do is multiply the focal length, not correct a sharpness problem.

Have you some existing footage at full zoom I can check out?

And before you do that, what kind of support are you using for the camera, i.e. tripod etc?