Have you considered using a 1/4" bolt? Those rivnuts would be easy to find. (1/4" = 6.35mm. The topcap is the only place where the bolt has to fit and it will be no less than 6mm. Common bolts are considerably under the nominal dimensions so I'd be surprised if a Home Depot 25 cent bolt doesn't just slide through. Maybe go an Ace Hardware and look though their selection to find a bolt with a head that will look good on the bike. That will probably drive the price up to 75 cents

To take this a step further into weight-weiniehood - make an aluminum bolt. I haven't figured out uet how you fabricate/attach the head, but getting there is half the fun. THreading 1/4" aluminum rod wouldn't be too hard and the end result would be light. (I'd set the headset and stem with a steel bolt, then replace it with the aluminum one to spare the threads.)

Ben.