I've changed to nylon now which I bore out to 6mm and glue and pin with 1.5mm precision steel dowels. Strangely this nylon is 8.3mm in dameter - don't know if that is an imperial size but boring out the holes to 8.5mm works well with the clearance issues you get with fitting stuff like this. I have to say that this is the best solution I have found - tuners designed for guitars though they can be made to work will make the headstock quite heavy unless you machine huge slots which make the headstock 'saggy and old' if you get my drift...