You don't really need circles, as darketc... said, but it's not hard to make circles. If you have anything that's the size circle you need, (like the lid off a different reactor) you can tape it to a piece of acrylic with double sided tape and flushtrim around it on a router table.

Reactors are pretty simple to make. I make a box under the tube so that the fittings don't have to go through the tube, a common leak point.