After mounting untold hundreds of such tube/rim combinations, I can assure you the metal plate is intended to go OUTSIDE the rim, with the nut holding it in place. If the rim is too thick, just discard the plate and nut. It's not there to keep the stem airtight in the tube, or anything like that.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!