Adapter plates are easy....
go to home depot or similar place and get a piece of clear (or whatever) flat plexi about an 1/8th of an inch thick.
While you're there, go to the plumbing dept. and get some plastic or rubber spacers or washers that are about 1/4" or a little wider.
Get some thin machine screws with nuts and some washers. Like the kind the 3 pt. or generic throw off are held on by.
Cut the plexi to about a 3" square or so...after your read this you'll see what I mean..
Mark the plexi with the hole mounting pattern of the clam shell and drill it for those holes. Then mark it for the mounting pattern of the throw off you want to put on your drum.
Attach the new throw off to the plate.
Then, put the clam shell screws through the plate, put on the spacers, and screw the whole thing on the drum.
Voila! Clear plexi looks best 'cause you can see the finish on the drum.
This works well and will last forever. And you don't have to drill the drum!