To make a 720K floppy for use on the Amiga w/ CrossDos, do the following. Take a standard 1.44M floppy and put a piece of tape covering the hole in the upper left side. Insert the disk into any PC using any version of windoze, and type the following from a command prompt:
format a: /t:80 /n:9
Thats it. This disk can be read by your Amiga as long as the file "PC0:" is in "system:Devs/dosdrivers/" directory (on the Amiga, of course).