My solution for the problem of stacking multiple filament reels on top of each other from lots of different brands.

It's basically just a customisable adapter that you can slot into the middle of each reel with a standard "stud" on the top and matching hole on the bottom.

I added a hole running all the way through the middle so that a strong piece of string or similar can be threaded all the way through the stack (with a knot or similar to stop it at the bottom) and wrapped around the also included handle for lifting the stack as one unit. I've used a length of filament in the pictures which works ok but after about two unwraps the brittle filament breaks and needs to be replaced.

I place my filament reels into medium sized vacuum bags when not in use and I found that the relatively sharp corners of the stud on the top reel could easily puncture the bag so I made a quick safety cap to prevent this.

To use simply open the scad file in OpenSCAD and change the parameters indicated for your own reels.

I tried various ideas in order to reduce the amount of material and time required but started losing strength quite quickly.

Note: this was my very first time using OpenSCAD so don't be surprised if there is a much better way to do it.