Hmm, i just checked ebay and it does seem like those mountings go for quite the penny.
If you have a bench vice i'd suggest you could cut out some 1mm thick sheet metal, then place a large steel ball in the middle of the plate, put a block of wood behind the plate and press, but not too much so you don't tear the metal, this will give you that indentation that you are looking for, at least i found this method to be quite useful in some situations.