You could also consider using a different adhesive, there's no reason you need to just use whatever's on the back of the labels you're using. Maybe a quick swipe of a Gorilla Glue stick would take care of things for you.
I have used a similar Brother printer for labels, black lettering on clear labels. I had 2 issues with labeing Vario binders- first the spine is pretty small so I was very limited in what could go on the labels. Second, I could not easily get the labels to be square and neat looking.
So what I did was put as much text on the label as was needed and affixed the label to the top of the slip case. The labels were much less noticeable so neatness didn't count as much.
Brother label printer using a "gold on clear" cartridge. Looks great, I haven't had any problems with long-term adhesion. Even if they started to come loose, it's easy enough to print another one and replace it.
As mentioned, Lighthouse does sell packs of country labels in gold lettering and I've used those as well. You can get labels for pretty much all the major countries, but less popular areas or specialties are generally not available. The Lighthouse country label packs do contain some plain Roman numerals you can use as well if you simply want to number them - perhaps referenced against an index.