Step 1: Required items
one hot knife, or soldering iron with a blade attachment
Step 2: Cut a pie slice out of the bottle
Begin by cutting away from the opening vertically downward with the hot knife.
The opening of the bottle is made of much thicker plastic, so this part takes patience.
If the bottle is dry, it will go faster.
If the interior of the bottle is wet, it can take an agonizingly long time to cut.
Apply very little downward pressure, just let the heat of the hot knife do the work.