Having just had a new chimney and flue built onto the house I was left with a spare length of chimney liner. We used the Schiedel Isokern system which comprises various diameters of basically pumice tube. It’s soft enough to cut with a hand saw and with a few additional bits of vermiculite board all sorts of arrangements can be made. I think it must have cut the heating time by at least a half as well as making everything much more comfortable due to the heat being contained. The liners are not cheap at about £25 a length but having used it I would not hesitate to go out and buy another length specifically if I had to do a larger boiler. Just make sure you get an inch or so bigger than the boiler barrel to clear any bosses etc.