The pop top Westy's had a factory made hole that was much the same as a sunroof model - I understand the drain channels/holes were not fitted on the Westy's - though the opening is the same size and location. So this would be an easier conversion if you found a good sunroof bus and stuck a Westy roof on it. Then just piece together an interior.
Also the Westy and sunroof models had belly pans fitted to compensate for the hole up top and to add rigidity to the frame.
Though as the chap above said, the right one is out there! I think if you want a pop-top, best find a good one rather than cut a tin top.
Hope this helps.
