Advise on how to sew/hem circular openings.

Just rolling the hem is goig to get bunchy unless your fabric is pretty strectchy, since folding it back means trying to make a smaller circle match up to a larger one. I suggest bias tape binding on the edge, or a stretchy binding- though with the stretchy stuff it`s tricky to avoid stretching it as you go.

Yeah, if you try to roll it, the diameter of the opening will get larger along with the perimeter length, meaning you will be trying to stretch the edge quite a bit.  In clothing, circular openings like this (arm holes, collars) have little cuts in them perpendicular to the edge that go up to the seam.  The cuts are made every 1 inch or so, and then when they are folded back the original edge can ‘expand’.  For an opening like the one you want that will have the final edge exposed, that would not be a very clean technique though.  I agree that a double-fold type binding would be the cleanest.  You don’t need to use commercial bias tape;  you could just select a heavier fabric of your choice (to have the edge be stiffer/heavier like you said) and make a bias tape yourself.  Cut it 4x as wide as you want the finished edge, then fold each outer edge to the center line so they meet, then fold once more on the center line.  I think a 1/4 to 3/8″ wide final dimension would work well for this.