i use a tube notcher that fits in a drill press

like this one



i user a cheaper verson i got off of ebay tho
the cheaper ones have an issue with striping the threads out on the hole saw and the brass bushing wears out but i bought it for a single project and didnt expect to build a bunch of these
i modified mine so it dont strip the threads out now and the bushing seems to be holding up good so ill see how long it last before i buy JD2's verson

Thanks for the compliments