I have been putting loops right in my fly line for years. You can make them really small to.



A needle knot would be good also. Attach a leader and when the leader gets too short, cut it off, put a perfection loop in it, and you have a connected butt section.
There are tools out there to make needle knots easy to do also.
I have the Shimazaki, but some lines do have an inner core that prevents a needle, sooooo, the line loop.