Canvas tubes would be invaluable for some niche buildings or tunnels, specially because there are few hollow canvas (the only one I can think of its the canvas dome). It would be a nice plus to be able to adjust the thickness of the tubes.
The tubes would also be really good for tunels of all kind, because right now they have to be composite out of canvas cubes, canvas cylinders as guidance and other stuff, and its very tedious to align them properly, specially because they’re somewhat buggy to copy things onto at the bottom.
Please, they’re a pain to align properly.