I use blue sleeping mat foamed stuff to make my gaskets. I use it for
all sorts of things including brew tub and column insulation.
They will collapse after a while however they are quick and easy to make and if you put a couple of old ones together they work as good as new.
You will need to tighten your screws after a while as they flatten with use.
A craft knife is good enough.
If you push your flange down it will leave an indentation to use as a pattern.
I use a sharpened tube to twist out the holes.
The craft knife for the centre then a light touch with the cigarette lighter to smooth the edges.