Right - see if I get any better tips for this one. ;-)
Made up the plate. The output of the valve is a cast ally pipe of 22mm OD. Not as perfect a pipe as, say, 22mm copper as it's part of a casting. Really meant for a hose, but I'm poking it into the plenum direct. There is already a larger hole there for the old idle valve.
So what I need to do is seal between the pipe on the valve and its mounting plate, to stop air getting into the plenum.
It would be nice to allow for easy dismantling if needed.
I wondered about fitting an O ring? Could make a second plate to sandwich it. But how to cut a suitable rebate in both those plates to contain the O ring and allow it to expand when they are clamped together? Or should I just use a sealant of some sort?
Think is it will be visible - more or less on the top of the engine - so body filler or whatever wouldn't look that good.
I suppose something like a rubber grommet that the pipe is a tight fit into might do - but the mounting plate is 6mm thick.