[would like to not drill a hole in the butt for the other sling swivel. ]
I'm sorry that I can't help you with the leather butt cover - but I have used another alternative.
You could obtain an additional buttplate, and file or relieve a bit from it's lower inside face, from the lower BP screw to the toe of the stock - then slide the end of a leather sling in there, far enought to be secured by the lower BP screw.
The leather that sticks out from under the buttplate can be either the sling proper, or a sewn-in QD, or connect the tab to the sling end with a Chicago screw.
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