I too have this problem, but I've found hopefully a solution with parts available to buy and detailed pictures as well as input via customer reviews on said part.
Here is the Amazon listing that I'm ordering today and is at the time of me writing, Prime eligible(Good to have fast free shipping)
[https://www.amazon.com/Dogxiong-Battery-Housing-Adhesive-Replacement/dp/B07B9H58D9/ref=sr_1_1?s=wireless&ie=UTF8&qid=1524851628&sr=1-1&keywords=Dogxiong+Black+Back+Rear+Battery+Housing+Top+Upper+Part+Really+100%25+True+Genuine+Glass+Camera+Lens+Cover+%2B+Adhesive+Fix+Replacement+Parts+For+Google+Pixel+2+XL&dpID=31fnO2DRbeL&preST=_SY300_QL70_&dpSrc=srch|Google Pixel 2 XL Upper Black Back Glass]
Gentle heat is recommended.
I hope this helps anyone else. By now this part is still a bit hard to find, let alone a quality one.
I'll post an update with my experience in detail with pictures included.