If you do some rafting you probably have some dry bags. Take a look at the available backpack style dry bags made out of ripstop nylon and silcone. They are very light and may be big enough for a hard concert case. In Australia the main brand is Sea to Summit for this type of product.
Also if you are crafty or know someone who can sew, the design of a simple silicone nylon back is very simple and the fabric is not expensive in the USA. You make a simple sack to fit the concert case, not unlike a pillow case pattern, and sew D rings on each of the 8 corners. Attach the shoulder straps to the D rings. For a closure, lace some para cord through the D rings on the top, and or fit some eyelets in the top seam. The bag should weigh less than a few ounces and will scrunch up into a small space for storage. Being ripstop nylon, it will also be very strong and resistant to tearing. Note that I use D rings and eyelets a lot to avoid the weight and effort of sewing zips and hook loop closures. The uke is already in a hard case so you only need enough closure to stop it falling out of the sack.
If you want a "professional" kit format, check out Ray Jardines website. His back pack kit will take an experience sewer maybe two hours or less to make, and it weighs around 800gms. The kit I made will hold a concert hard case easily with room for stuff like a water bottle, music, etc.. Note that you can look at the design of the kit and leave the outer mesh off, and a good sewer will be able to make small adjustments, so that the backpack is aeroplane locker and security friendly. Or you can make it exactly as designed and get some good use out of it as a day pack or hiking pack.
I also put hard cases in "normal" 50 litre dry bags on wet days. They have a shoulder strap and room for other stuff.
