They are only made of mild steel ,
i had to drill a few extra holes in a brattonsound one , no problem at all , just use a normal metal drill bit.
Well i tried to drill mine and it blunted 3 new quality normal metal drill bits and half deafened me with the screeching noise before i gave up. mine was having none of it!
no dissy intended with me reply but there is not much a decent bit will drill.