This was for a JW or Lightwin .Using metal core gasket material. Really hard to cut by hand.
Photocopy old gasket on desktop printer. Spray glue on to 3/4 inch plywood. If you don’t have a gasket you can copy the cylinder cover. just pack towels around to keep ambient light out.
Screw two pieces of plywood together with stencil on top.
Use drill press to keep everything square.
I used a Forstner bit to cut the cylinder holes. Spun it in reverse at first , then in forward thinking it would cut cleaner. Used drill bits, and hole punches for all the water passages and bolt holes. I’m guessing a decent hole saw of the correct size will work too on cylinders.
Sandwich gasket material between plywood and locate opposite corners with bolts and nuts to hold the gasket firmly. Then do the rest of bolt holes and bolt them together before cutting out the cylinders with the Forstner.
Maybe a router would work too.
I have attempted to make one gasket out of this material. Whish I had seen
your procedure using the plywood first. Will definitely try it if a attempt to
make another. How did you remove the excess material from the outside of
the gasket.
Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.