New 20 ounce tumblers available now! Forum donation credit with purchase. https://www.wildguzzi.com/Products/products.htm#Tumbler
Not the same, but an option. https://www.mgcycle.com/index.php?main_page=product_info&products_id=373-AJ
I think It's the same p/n as the loop frames:: 55 43 65 25. You could probably make one pretty easily. I think the big challenge would be to find the rubber grommet that fits into the wire loop.
I believe that all of the Ambassadors or Eldorado's had the all rubber guide that requires fender drilling (not 100% sure on that). Rubber grommets come in many many sizes so I don't think that would be a problem. I don't think I'd be very good at making a wire one, especially since I don't have an original to pattern it from.