I machined mine out of aluminum on the lathe. I epoxied them in and the ground them flush in the port.
Ahh...well, I'd love to have that option...but no-go...and machining something locally would just set me back several weeks.
OK, for now I'm going to play it safe and just attempt the valve-cover bolt clearaning in the roof and nothing more. That should be plenty enough...right now this is what my blending of the guide boss looks like:
