Many of you have recommended it. I finally have some. I wish I had gotten it a year ago. It was cheap. It weighs almost nothing. It is machine washable. I can easily use it to make my clown tophat stay up, and to make the brim stiff, and still be able to smush the hat up and stuff it in my backpack without damaging it. I may open up the lapel seams to insert additional buckram in order to keep the crease stiff. I may also use it top keep the pleat in the back of the jacket.
Now that the jacket is more than half done, I have finally found a jacket to copy. A marine mess dress jacket lacks tails, but is otherwise exactly what I want. I think I can still make a decent copy.