Spreading the dropouts works ok with steel but at times can be a PITA. Don't know about bamboo. I really don't see a reason that the aluminum block couldn't be replaced with a hardwood block that has the the correct spacing. I'm assuming that you have some woodworking tools since you are working with bamboo. A good quality drill press and a table saw would get decent results I would think. Shimming the rest of the fixture will work as well. Just make sure the other fixture parts are well supported across their width so the can't "wobble".