Step 1: Get a rally big plate!
Now trace the bottle mouth (top cap removed) ,underneath the plate in the center. I used a felt tip marker. Get out your trusty Dremel Tool fitted with a composite cutting wheel, (please: wear gloves & eye protection ) and carefully cut out the circle that you have just traced. Cut inside the line for a good fit. You can always remove more plastic for a nice fit but you can't put it back! It does not have to be perfect, as long as the bottle threads fit through the hole and the cap screws onto the bottle from above with the plate in between.