Or.....
Just 2 cuts are needed....much neater
.....
Make 2 straight cuts North and South (12-o-clock and 6-o-clock) from the INSIDE of the opening, cutting through the thread and as far back as you need to go.....you have now created your 'flap', Pull the flap back enough to insert the pump...push flap back down...and screw the lid back on.
You can use some tape to seal the 2 cuts.....makes it look tidier too
.