Circle Hollows

Ok, so the ability to take a flat square and hollow it out is very cool. The ability to alter the shape of that hollow is equally cool. However, the "circle" hollow is not very "circular". Is there any way to get this to do a real circle instead of this jagged "it's a circle if you squint really hard and look at it from a 27 degree angle" thing?

I want to build a simple door arch. Now I know I can take a cylinder, hollow that, place that in my walls, then add triangles and blocks around it to patch in the gaps. But this is not only object-expensive, but it blinks and flutters when you look at it (due to the objects overlapping and trying to get their draw priorities out there). What I have done in the past is take a flat square, cut it's ends off (so it is just half a square) and then hollowed it with have a "circle-ish-looking-thing". While it does the job (and does so in a single object), it's not pretty. Not only that, but there is no way to get the "circle" straight...it is like it is tilted on a edge, so the arch portion itself is not even symmetrical at both sides.

Can we do better with this, or is there a better way to do what I am attempting to do?

You could also use a hollowed cylinder to in addition to the hollowed cube to kind of round out the icky cube-hollow's edges. But, this still does use two prims, and you will still get the flickering...

My suggestion: do what casval says but make the half hollowed cylinder thicker than the wall. It will look good and no flicker.