Are you looking for a rubber grommet or a strain relief? The company that makes so many of the snap-in strain reliefs that their name has become generic for the product is Heyco. They are specd by range of cable size, diameter of panel hole and thickness range of the panel. If you go to their site you should find the one that fits. Then google the part number and you may luck out but electronic distributors like Mouser, Arrow and Newark carry a lot of their line, even Amazon has some of it, as do some surplus places and ebay. Ideally, the hole should have two flats to prevent the relief from rotating but a round hole will hold fine. It is not a hand tool and nobody is twisting the cable and they usually grab pretty tight, anyway, if sized correctly. 3/16 to1/4 should not be a problem. If you can't find the info., post the wire diameter, exact hole size and thickness and I will see if I can come up with the number.
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