It turns out that a 5mm flat washer is a pretty good match for the dia. of a stock 6mm bolt head. The ID is about 5.3mm, so it needs to be opened up a bit. I used a small grinding bit in a dremel tool to try one.

Of course if your stem uses a bolt with a reduced head diameter, then you'd probably need to reduce the washer OD also. FB and fietsbob had some good suggestions on how to do that.

I don't have any 4mm flat washers, but looking at the specs, it looks like it is pretty close to the dia. of a 5mm bolt head.

Looking around, I noticed that Bike Tools Etc carries some 7mm bolts, not stainless, specifically for stems. That's the only source I've seen for any kind of 7mm bolts.
http://www.biketoolsetc.com/index.cg...em_id=US-CS725