I've been wondering the same thing and have been thinking about wrapping a tarp around the chimney and securing it with a bungee cord. I don't think it would be a big deal to remove before using the sauna.

Like @andy said, disassemble the chimney when not in use.

Also: I wouldn’t use a towel as a fix, you’ll start a fire.

Jjp! the only thing i would recommend is when not in use take part of the chimeney down and run the flap back down the square black flap on the roof for the hole. To prevent water from coming in and being all over your stove. I would recommend wrapping some sort of towel or something around the pipe outside while it rains, but that might cause some added weight on the roof and it could cause it to fall or through the are strong, but they’re not designed to hold a lot of weight up so I would go with my first suggestion. Hope this helps. Let me know if you have any other questions.