Everyone knows gyroids are cool, don't they? Flexible filaments are cool too. Combine the two and you get a a sort of tough open-cell foam. I was trying to come up with a practical application for this, and found inspiration in a pair of worn out shoes.
I copied the outline of the existing innersoles and created an approximate foot profile to sculpt the upper surface. The result is actually quite practical; good springiness and very breathable. The files are for approximately US mens size 10, (EU 44, UK 9).
The gyroid structure is not part of the model. You need a slicer that supports gyroid infill, and you turn off all top and bottom layers and sidewalls. (I used Silc3r Prusa Edition, which works with just about any printer, not just Prusa's.) Adjust the infill density according to your requirements, and how soft your filament is. I found 30% works well with TPU, and I turned the extrusion ratio down a little.
I'd call this more of a starting point for anyone who wants to experimens, rather than a finished item. You can probably achieve quite a lot of customization in your slicer by adjusting x, y and z scaling and trimming the print to size with scissors. I have also included the Fusion 360 file, which is very easy to customize. You can take a photo of an existing inner sole, load it as a canvas and adjust the outline to match the profile.
Also note; I have no orthopedic qualifications, this model is provided without warranty, and don't blame me if your feet fall off!
I hadn't seen this previously, but someone has already put together a funky insole model generator with variable density capability. It looks like it predates gyroid infill, but I think you should be able to print those with gyroid infill just fine.You'll get the variable density along with the added ventilation from the gyroid structure:
Doesn't matter Flexible
Gyroid - disable all solid walls