I am using an Amazon sourced Motobatt MBTX7U in a fiberglass case from Classic Bike Shop in UK. It is an 8 amp 12v AGM and is a very good fit, leaving room for a fuse and some connections inside the case. I don't think it would fit an empty standard case, but this thin walled one is perfect. I looked a bit for a case that was the right size internally, as they seem to vary a lot. Who would want to see something so yellow in plain sight, so a case or paint is a necessity.
Ron
Ron