No disrespect to anybody who makes their own disk, but I think the point is to give a reliable reference point from pond to pond, anywhere in the world. If you make your own, it would have value for your own personal use to compare water clarity from one point in time to another, but would have reduced value when speaking to a biologist who uses a standard Secchi. For example, if you were to want to make a decision on liming, or alum application, and was giving information on your pond to a specialist, the home made secchi wouldn't give as reliable information as the standard. Just being picky, I guess. Sort of like telling Lusk that your bluegill are as big as your hand. It would give him a general idea, but everybody's hand is a little different size.
