I don't know if it would work for you, but have you considered something like 2.5" schedule 20 PVC pipe? You could buy a unibit and drill holes in the tubing to further reduce the weight while maintaining the structure of the tube to protect your rod. Buy a couple end caps and glue foam to the inside of them and custom fit the tube to your rod. Just a thought.
Look on eBay or Amazon for polycarbonate tubing. Available in 6 foot sections that you cut to length. Various outside diameters available, 1/16” walls. I had to hunt around for end caps, ended up buying them from a company in Prescott Arizona. Pm me for more details.
Friend went ultralight on a tube and ended up taking a tumble and breaking the rod! So, in my mind I would think that there are a lot more ways to save weight than the rod tube. I carry a gerber dime for a multi tool. Don't use it much, but 2 ounces vs most others at 16 ounces. I never carry water, just a sawyer or katadyn water filter bag. Fill it at the creek and keep walking. water is heavy and where I hike to there are fish, which means water. I only carry one fly box, fish at high alpine lakes don't see many flies. One box with a dozen or so dries on one side and a dozen or so nymphs on the other. One 4x spool of tippet and one spare leader. I carry a pair of forceps with scissors, not both. I carry an anchor bag with a 30ft piece of paracord, but no other rope. One rod and one reel. My packraft and paddle. And a very light weight pair of wally world water shoes.
With the backpack and everything, I am at 24 pounds. My rule of thumb is if the extra item I am carrying does not have at least two uses, leave it in the garage.
+1 on making your own rod tube out of plumbing pipe and fittings. I've made a number out of the black ABS stuff for my single and double-handed rods. I don't know how lightweight they are, but if you're making a tube for only one single-hander it wouldn't weigh that much. Also, those ABS tubes are basically bomb proof, much stronger that even the standard aluminum rod tubes.
If you make your own I suggest putting a number of fishy stickers on it--makes the tube look a little less "threatening" if you're travelling through airports with it.
This is the best compromise of weight, strength and cost that I've ever found, it may not be called Schedule 20 at some lumber yards but ask for thin wall drain pipe.
Another lighter weight tubing that is less available and more expensive is fiberglass (or carbon fiber) reinforced plastic tubing; do a search for "FRP tubing".
A problem may be the 2.5''- most uses seem to skip from 2'' to 3'' so you may have settle for 3''. Because PVC pipe is sized with the exterior dimension the thin wall has a greater inside diameter than the common Sch 40- what this means is for my two piece rods 1 1/2'' SCH 40 is needed but 1 1/4'' Sch 20 works at about half the weight, point being that 2'' thin wall might work for you?
ABS vs PVC- I think ABS will be heavier in the same size and ABS will deform in sun light much worse than PVC, ABS should withstand impact slightly better than PVC but may have enough flex to be a concern.