Not a Boston native (trying to relocate there!) but there is public transport in my city and I have a bus pass. For me I like to slide my pass in a card holder, that I hook onto my bag's key ring! I did the same when I visited Boston last year and took the T.

This one from Muji is great and can hold two cards: https://www.amazon.com/Muji-Soft-Metro-Card-Pass/dp/B00E5YANOK/ref=asc_df_B00E5YANOK/?tag=hyprod-20&linkCode=df0&hvadid=167144043525&hvpos=&hvnetw=g&hvrand=4420826060118171735&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9005169&hvtargid=pla-305497048280&psc=1 You can find it in their physical location on Newbury Street too :)