I looked into this fostnerbit, very expensive, as recomended in the manual for these horn mounting blocks, but will instead use a sharpend brass tube (of proper diameter) to drill/twist/cut a neat hole into the foam core where the balsa bit will be inserted. After this I will follow your advise of masking the exposed foam while shaping the balsa insert flush. Then mark on my plans its exact locaton before sheeting. Afterwards one could just use a needle if uncertain of the horn mounting location (if forgotten to mark its position on the plans).