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I personally have never seen anything for this kind of application.
If you send me a picture I would be better able to answer. But having said that, what is used is acme non tapered threads and a jam nut. The jam nut would be brass and the trim inside the tub would be a 2" shower drain having a 2" PVC or ABS socket. The fabrication is one that requires some skill as this is not a standard set up. There are ways to fabricate 2" IPS from a shower pan drain fitting. Or using a section of 2" threaded PVC and a 2" jam nut on both ends and a drop in grill for cosmetic reasons and to keep out bars of soap etc. The flush valve of some toilets is a 2" acme thread use ta nut for that for jam nuts and use the spud gasket for the seal.