I thought about this all night and decided to tap the hole to 12mm. The keensert looks great but I don't think there is enough room to countersink it. The tread depth is only about 10/15mm so I wouldn't want to take away any strength.

Thanks for the input. A lot has been going on here and I couldn't think straight. I didn't even think about the helicoil leaking - Doouuuhhh!