The thing about a simple, shop made plate is that you will find it hard to use inserts, which can be changed to fit your different sized bits. Having the insert fit close to the bit size helps with above table dust collection. Suggestion on cutting the plate opening is good. As to cutting the opening for the bit, it needs to be a little bigger than the biggest bit you expect to use, which means for all smaller bits, you'll have way to large an opening. This is why most of us use commercial plates, some of which now have twist lock inserts instead of the 3-screw (where'd that darn screw go?) veriety of old. Kreg makes some nice aluminum plates for reasonable prices. And they have already rounded corners.
The more I do, the less I accomplish.