Many many years ago, I wired up €œcooker€� to the cooker side of a cooker & 13A socket affair. It was in a council house and €œthey€� had installed the cooker switch and then there was a very beefy cable outlet behind the cooker.

My €œcooker€� just required a 13A socket for its spark doobry and clock.

I was surprised to find the outlet behind the cooker
(Basically a back box, a hefty brass 3 pole in insulated chock block and a cover). Had no wire coming down from the switch unit.

From memory, I took a thin gauge flex from the cooker switch down the conduit to the connector and past the chunky choc bloc and wired straight into a surface mount 13A socket.

You could use chunky T&E down to the choc bloc (assuming these accessories are still sold) and then
Your 15mm dia stuff from there.

Cheers

Chris