External Product ID and ID Type for Handmade Product

I already have an eBay and an Etsy store, and I’m trying to list my handmade item on Amazon for the first time. I’m trying to sell a ring which has variations for each ring size. I finished most of the listing but I’m stuck at the section in variations where it wants an “external product ID” and “external product ID type.”

At first I was just going to make up a number for this, but it only allows you to choose “ASIN, EAN, GTIN, and UPC.” The problem is that I have about 18 different sizes that I want to offer, so I don’t want to register each one of these variations with it’s own UPC etc.

What is the best way to handle this section? I seems kind of ridiculous to me that you need registered barcodes for a handmade product.