I’m not real familiar with it, but you might look for some chain with a width wider than one link. Seems like that would provide you behavior more similar to belts. It seems McMaster sells double link width, but there may be other sources of finer/wider variations.
https://www.apple.com/iphone/compare/?modelList=iphone-SE,iphone-SE-3rd-gen
For 10mm difference on the height and width, it’s time to buy her a new iPhone as well. It’ll run the latest software, and have much better battery life than her worn battery.