3mm holes are perfectly fine, however use m2 2mm thick screws as some older cards use smaller screw holes for the heat sinks.
the only way i know for the square pumps is the old dwood square but i think that was even more limited due to size and it being sat on the card.
i decided to do a universal approach for both style pumps with my 2 point bracket which works really well and gives alot of flexibility to the user. because i cant cut mine myself i had a large batch made and i would say do the same.
if you need any help/any questions send me a pm