If you don't mind the thickness of the finished 'decal' this is an excellent candidate for inexpensive die-cut self-adhesive vinyl film, which would be completely opaque white, fairly easy to apply (use soapy water and a squeegee) and there are many suppliers in nearly every town.
OTOH, if you want thinner, "real decals" your choices for opaque white ink on (either) a clear vinyl carrier or waterslide are much more limited.
But I'd say try the usual guys and I'd highly recommend this guy (Josh) for the waterslide type, plus I'm pretty sure he will work with you to create the vector art needed to print this (for a price): https://www.bedlamcreations.com

Beatutiful frame, BTW...I almost snagged one of these off Craigslist, years ago, but...no dice.