Alex,
It's your boat of course, but I'd be reluctant to cut holes in my boat, especially for something as ineffective as those deck top dorades. I think I'd look for something to fit the application rather than modify(cut) the application to fit a replacement part. I replaced mine with 4" solar vents which fit without modification. The dorades are only unidirectional, don't seem to move much air, and have an annoying tendency to catch a jib sheet when tacking. The solar vents can draw air from all directions, have a fan to help move air, and have a low profile which won't foul the jib sheets.