From looking at, running and measuring K-Drill and Clam optional accessories it looks like the K-Drill items are a bit more pricey but are mechanically spot-on size-wise. 2 of the 3 shafts I bought for my Clam-Plate (from Clam) had runout beyond what I deemed acceptable.
“My opinion only” buy your adapter for a K-Drill from K-Drill.
K-drill adapter Clam Plate to K-drill is $14.99. It’s the top one in the picture. Looks to be available from Cabelas @ $14.99 with free shipping using the code 7HOLIDAY.