I love the original model by @spicy3d and like many others, wasn't happy with the extra room in the compartment that allowed the batteries to roll and rattle around. So I threw it in CAD and added a sleeve to fit my batteries exactly. Not too tight that I can't get them out and not too loose that they rattle around.
I'm not sure if this size will fit every 26800 battery out there but I'm happy to modify my remix and post it up if you need different sizes. The holes are 27mm. It's a good middle ground and should fit most batteries without rattling. If you find that yours don't fit well, then measure the outer diameter with some calipers and let me know. I can go up to about 27.6 before it's not a useful change as larger batteries won't fit without enlarging the case as whole. I can go down to 26mm. Smaller also wouldn't be useful for this size battery. My batteries, with the protective sleeve are just at 26.7mm. The extra .3mm allows for easily removing them.
I uploaded models with and without the battery designation (26800) as text recessed into the bottom just in case you don't want that.
The lid is the original. You'll want to print it on it's side with supports for the mounting tab. Printing it flat makes the opening tab weak along the same plane that you push to open it making it VERY prone to snapping off.
Printed on a Bambu X1C in black PLA-CF:
The author remixed this model.