I have a square one, it's just held in place with nut, bolt and washers, screwed straight through the bumper so probably not the best way of securing it if you have a r32 bumper or a nice anni, mines a bit of a track car so not too worried about cosmetics, I chose a square plate just because I cut a great deal off the bumper so had to get a smaller number plate to be able to mount it on what's left of the bumper.
You could look into plate holders for evo, most come with a square plate from factory
