I'm going to give you a few things to try in hopes that one of them will work for you because depending on circumstances you may or may not be willing or able to use them:
If this is a keyword important enough to merit special attention (it sounds like it is) then make sure you split it out into its own campaign(s).
Getting the keyword into the ads
1.) I'd try creating a "Dynamic Search Ads" campaign...you don't choose keywords, you just tell Google to use the content of some or all URLs on the website and to show your ads on queries it think are relevant and Google writes the headlines for you...if the person searches "plastic windows", the headline Google writes may use that term but you can avoid having ads that include the term.
2.) I'd also create a campaign that is keyword targeted but that uses Dynamic Keyword Insertion for the same reasons as above (then see which campaign performs better)...you could use DKI to insert those keywords in a headline, the description and even the display URL.
Getting the keyword onto the page
3.) I don't think the Google Ads Bot factors meta tags into QS but could you rename assets like
<img src="plastic-windows-and-pvc-windows.jpg" alt="plastic windows is a term sometimes used to describe PVC windows">. I'm not sure if filenames and alt text are factored but I usually optimize them anyway and they aren't "visible" unless you look at the source code.
4.) If you can get away with changes that are only visible in the source code, then you could try (no guarantees, try at your own risk and make sure the pages are set to
noindex so you don't hurt your organic rankings) old school, on-page, blackhat SEO tricks like making text the same color as the page background.
Manipulating how QS is calculated
5.) When/if you create those new campaigns consider creating an "easy" conversion (use a conversion metric like a 10 second phone call or one that fires on an event from Google Analytics like a "scroll" event or a "click" event)...that way you get a high conversion rate (at first), then as time progresses you create new ads in the same campaigns that use the standard conversion metrics.