thanks - two comments:
As for the mast size: That is the crux of this design - it must fit tightly to the mast at the appropriate height above ground. You can use rubber bands to make it stay at the desired position with rubber bands if the mast is smaller. A more elegant solution would be a set of adaptors that match the mast diameter to the antenna coil diameter.
As for the coil position higher above the ground: I tried that, but the gains in performance where insignificant, and the loss in ease of handling were severe. So I abandoned that idea. I think that resonant elevated counterpoises make for a much bigger effect than the position of the coil.
73 de Martin, DK3IT