Really? Huge? Come on now, since when is a 1/2" tall slot huge? Do it correctly and there is no reason to be any bigger than what is in the picture.
The grab bar also gives you two additional frankenstein bolts that come in handy to tie down luggage and stuff. The bar can also be very useful when you need to push the bike around in the garage. And at sometime you will end up finding yourself parked down hill in a parking spot and that bar will give someone a solid hold to pull you backwards out of the parking spot. If you don't have the bar then think of what piece of plastic is going to be grabbed on to pull you backwards?
To the OP. I just eyeballed it, no tape, no markers, just a dremmel and 30 minutes. Done deal, one cheap mod that I use often.