As far as sealing , grommets didn’t work, lot of oil mist. Went to a rubber gasket & o- ring with metal ring fastened on top.
So the chamber holding the plug became part of the "pressure vessel" of the motor? Now I see why peeps race triumphs. All this is done. Really informative thread thanks...
My thought was a built up tunnel with a machined surface on the cylinder headthat meets a tunnel from the top built down from the cylinder head cover with a groove with an o-ring in the middle for sealing ala Honda. The ignition wire would then have a rubber cover to keep dirt out. Two bosses fore and aft of the holes to provide sealing pressure with bolts. Alas expensive.
A second possibility was to go sideways with narrow D sized plugs through the cooling fins in the center of the head to get to the opposite cylinder and thus avoid all the through the cylinder head cover rigamarol. Has anybody done that?