These can be rounded in a lathe, either wood or metal. I use a metal lathe. I mark one end as close to center as possible and then drill a small pilot hole, just enough for the point of a live center to set in. I chuck one end in a 4 jaw chuck on the head stock loosely. I use a wood chuck like the nova and don't use any jaws. If it's too big, you can slightly round that end using a bench grinder. (This makes it easier to put in the chuck anyway) I then bring the tail stock up to the squared end that I've marked as close to center as possible. Tighten the chuck down but only enough so that you can still move the other end a bit. I then tighten the tail stock so it won't slide and adjust the live center into the pilot hole. I then got back to the chuck end and tighten the chuck down some more and then adjust the live center in till it's tight and spins when I turn the chuck. I then tighten the chuck down the rest of the way, but not so tight as to break or crack the lucite blank.
I then use wood lathe tools to rough round the blank. You can use the metal cutters and cross slide if you want to get it perfectly round, but I've found that for stem making or tampers, it's quicker, and round enough, too just use the hand tools. Once I get it fairly round as close to the chuck as I can get it, I turn it around in the chuck and then round the other end the same way.
There are other ways to do it, such as using a spur drive in the headstock rather than the chuck, but it works for me the way I explained it so that's how it's done in my shop.
