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1 You can make a custom router base out of acrylic in a rectangular shape to fit the channel.2 Make the end guide pieces longer to reduce the tendency to waggle. It's OK if they project past the slider ends when you take the router that far.
GFCI devices respond to current imbalances between the primary inputs which indicate that some current is being diverted to ground. They do not protect against shorts in transformer secondaries. That's a job for fuses or temperature sensors.
It's a spherical section as the pressure difference is equal over the surface.