Made some caps for cardboard filament cores to convert them to small storage boxes. (Like Elegoo & Polymaker) 

Made in Tinkercad!

I printed them in PLA on my Ender 3 Max a while ago. It was some try and error to find the correct sizes. I can't guarantee that it will fit your cardboard core immediately. You only have to scale the X & Y in your slicer.

I highly recommend you glue the underside to the cardboard core with superglue. While it's a bit tighter, it can come loose.