I upgraded the plastic bag I normally use for a first aid kit with a flat box. It's sized to fit the smaller Tegaderm* sizes, which is usually the first (and largest) thing I put in my first aid kits.
The file is designed to be printed with a flexible filament (the two ridges overlap for a secure fit). If you need other sizes or want to print it in other plastics, you can modify the design here
Sainsmart Transparent Red and Pink TPU*