Micro,
Al Drew's Music in Woonsocket Rhode Island may have the cast Knobby cover you need. They bought up most of the leftover Swivo parts when Rogers folded. These covers pop up occasionally on eBay, but the parts strippers are usually asking ridiculous prices for them.
Listen to Ploughman's advice regarding the set/set-pull screws. Set screws are generally intended to exert pressure at their base (like the set screws which hold swivo balls in place on Rogers tom mounts and cymbal tilters); they're not intended to hold down a plate with their tensioning head. I've used this DW screw on Rogers Knobbies, but I had to grind down the collar a bit as it was too large to fit into the Knobby's screw recesses...
[Attachment: 107066]
Good luck with your Knobby mount, and keep us posted if you have success.
Mike