Step 6: Cut the Top Hole
A series of small holes drilled around the base of the tank's threaded fitting is the easiest way to remove the fitting. This will create the start of the the tank's Top Hole. The Top Hole is where the top of the plastic bottle will pass through.
Holes are drilled under the overhand of the fitting's threads (right were the fitting's neck meets the tank). This ensures that the resulting overall Top Hole in the tank is not too large for the plastic bottle.
Use a small pair of cutters or needle nose pliers to remove the remaining web between the holes.
A file or grinder can be used to size and smooth the final Top Hole. It should just be large enough to allow the threaded portion of the plastic bottle to pass though.
The largest bottom flange on the bottle's neck should not pass through the Top Hole. The bottle flange should be the "up" stop; bottoming-out on the under side of the Top Hole.