I was thinking to file the bevelled faces so that they where smooth again and then the inside of each halve to make them smaller so that when I thighten them it will clamp tighter. But the shim is a much better idea just need to make it fit as @mdcc_tester mentioned.
Is there enough material in the two bits for you to file it to fit?
Not an expert on this and tester is probably your best man since he's been machining loads of custom stuff.