First- have you tried a necked down type of casing end ferule? It's the style that Campy used for decades and use to come in a small packet with many rolls of SIS casing. Shops often just give these out with a cable or casing purchase.

Second- There have been some CS stops that fit the common flat ended ferule, often actually about 5.5mm in diameter, that have been made and one time available from some framebuilding suppliers. I've seen others use a down tube or top tube stop on the stay, I use Aero single housing stops. – Framebuilder Supply due to their rounded shape there's a lesser chance of tearing skin on them

Third- Here's the chance to make something unique to your bike. A bit or 3/8" bar, drilling/cutting/filing can result if a homemade stop in little time.

Not asked about but here's my self made DT gear cable stops. They fit the common m6 threaded brake cable adjuster from DC or Weinmann calipers. Sub in a unthreaded bit with a stepped internal drill through and you have a simple non adjuster stop. Andy