I have a 140mm OD flue pipe which is about 5.5". As you say, there is no standard narrow boat collar for that size flue. I got hold of a short section of very thick wall pipe which was about 6.25" ID and had it welded into the roof making sure only a few inches protruded. It's been in place for over 20 years without a problem. I think it's at least 8mm wall thickness so acts as a heat sink like a collar. It doesn't corrode or rot so no issues like that. However, your next problem is finding a chimney (or a pipe to use as a chimney) which will fit your welded in pipe collar. But that will be the same if you get a bespoke cast iron collar made. If you can't find anything that goes over the pipe collar you could always find something that fits snuggly inside the pipe collar as long as you use a storm collar on your chimney just above. Also remember that the chimney and flue pipe are not supposed to touch each other, that's part of the job of the roof collar. You don't want any potential small movement of the chimney affecting the position and sealing of the flue below. Just make sure you size everything up before you start cutting holes in the roof, welding, etc, or before you decide to order a new bespoke cast iron collar.
