You'll need more space than your actual measurements for the hips as when walking you will need space for the movement - otherwise the fell will be constantly flexed.

This is not such a problem with a more flexible fabric, but with canvas you will need to make the fell wide enough to move inside of, or have your skin try to move under a tight layer of canvas. (ouch)

Look at what your measurement is when you sit down too - you wouldn't want the aprons sliding backwards and apart too often, which might happen if the fell is too narrow to go round when you are seated.