Hey Greg,
Thanks for the reply. I've tried some nylon sleeves that were 1/2" OD, but they're just slightly too big for the 12mm hole. That's why I was wondering if Cafetbird had a line on some specialty bushing... 12mm OD isn't exactly a common size for Ace Hardware.
I suppose I could cut a thin vertical slice out of the nylon sleeves so that they'd pinch together and be slightly smaller... might give that a try. There's also one specialty screw & bolt place I'm going to call on Monday.
It did occur to me to hit you up for more spacers, but I thought I'd try to exhaust my other options first.