I used a 4" circular saw blade. It left the surface rough, but I smoothed by hand with 40 then 60 grit paper. I left the seat loose the first half-dozen times I used it so I could carve the hot spots that developed after a few hours paddling.
Warning: I had to retrace my drive one morning because the loose seat blew out of the boat when it was on top of my car.

: To indicate when I've got down to 1/2" I plan to put 2 saw cuts on the
: bottom of the seat from the back to the middle. I hope this will work?
: should also provide drainage.

: TIA Terry