We call it sticky-back plastic in the UK. That or the vinyls used for sign writing and truck/van advertising come in a huge range of colours which would save the effort and uncertainly of paint – although i admire your industry..
Local eBay lists some suppliers, including smaller sample sheets and probably you could also find someone who would pre-cut small quantities.
I will admit to having used a similar approach when making a new post box for the front drive – except I used scrap UPVC from double glazing leftovers and cut and filed it to shape before painting with gloss and UV proof varnish.

pgk

Several decades ago I played with a cheap vibrational engraving tool and engraved a few images onto stainless steel, My technique then was to paint the steel with cheap white emulsion, carbon copy the main outlines, engrave through them, clean off with car cutting back paste and free-hand the rest.
Some sort of idea like that might work or finding a water-soluble low tack adhesive to fix the print before cutting then peel away.
Bribery is the other way – gin, wine or offer to knock up some nice brass corners for her scrap-books…

pgk