I bought several cereal containers for my filaments, and I wanted a built-in spool holder.

Design Goals and how I achieved them: 

• Minimal extra hardware
  • Filament stumps for locking the parts together.
  • PTFE tube for the spool to roll on.
    • The PTFE is held in place by surrounding walls and by threading filament through it.
       
• Use as little material as possible.
  • Only needs around 64g of PLA for a complete spoolholder.
  • Thin top and bottom but sturdy walls. 
  • The grate holding silica is as thin as possible because it takes the longest to print.
  • Printable with 0.2mm layer height, 2 walls and 3 top/bottom.

• Use different spool sizes.
  • Designed to acomodate spools between 100mm and 205mm in diameter.
  • Spool can take up the whole width of the box or be very thin.
     
• Optional Hygrometer (I don't need one for every PLA spool)
  • Option for round and rectangular Hygrometer.
     
     
• Stackable labels in 3 sizes with blank labels that you can add text with PrusaSlicer:

• Easy to open the silica box but stay shut if dropped.
  • Stick filament from side to side holding it shut and just poke it out with another filament to remove and open the box.
  • The recess in the bottom lifts the silica box over the cereal box's mold scar so it sits flat on the bottom of the box.
     
• Easy and fast to print.
  • No supports, 0.2mm clearances and mostly rounded corners.
  • Chamfers on all vertical slopes and fillet on horizontal corners.
     
• Modular
  • The Hygrometer is optional and stackable.
  • Modular so you can use the hygrometer you have if you want.
  • Filament labels can be stacked on or without hygrometer.
  • Readymade and blank labels.
  • Future modules are encouraged.
  • Fusion360 files.

Tested with the following cereal boxes but most others should work:

• Kichly Cereal Storage Containers (Set 6) - BPA-Free Plastic - Airtight Containers for Storage in the Kitchen - Dry Food Storage - Cereal Dispenser for Kitchen & Pantry : Amazon.se: Home & Kitchen
• GoMaihe 4L Kitchen Storage Box, Set of 6, Sealed Plastic Container with Lid, for Storing Cereal, Pasta, Flour, Pet Food : Amazon.se: Home & Kitchen

Inspiration taken from:

• FlexiSlide - Flexible spool holder by MiddleDane - Thingiverse
• Silica Gel holder for 4l cereal box by Daniel_Schenk - Thingiverse