What Do You Call This?

I need to clamp a round piece of tubing into a chop saw--and I need to clamp it on both sides of the chop saw blade.  (Clamping something round doesn't work very well, of course.)  My plan is to make two squares with a hole cut through each of the correct diameter.  There will be a threaded hole at the top of each square into which a bolt will go that will lock onto the tube to prevent it from rotating.  I will machine these rather precisely.

But before I make these to fit 2" and 2.75" tube diameters--is there anything like this whose name I do not know, and that is why I can't find it?  I would think the need to hold a round piece in position on a chop saw would be pretty common.

UPDATE: I was thinking about this, and I see a way to make something like very easily.  Buy square aluminum tube where the inside dimensions are the same nominal diameter as the tubing, and the wall is perhaps 1/4" thick.  The inside dimensions of square tubing are typically slightly smaller than the nominal outside diameter of the round tubing, so I can mount the square tube in my 4-jaw chuck, and use a boring bit to enlarge a few thousandths of an inch so that the round tube goes into the square tube.  Then cut the square tube into the two sections, and drill and tap a hole for the bolt that locks the round tubing in place.  Now I have something that locks the round tubing in place, and that will allow me to use the work clamp on the chop saw to hold everything in place.