Just in case you want or need to make a replacement for the bottom plate of the Ortlieb Vario PS 26L. Maybe it can also be used inside the 20L version with some scaling, I'm not sure it has the little pockets where you push this plate into on the ground.
I also provide a .dxf drawing, just in case you want to adjust it or use it in a CNC or laser cutter.

The plate consists of two materials: ~1.5mm thick plastic (not sure which material) and ~2.5mm thick EVA foam; so you can probably get away with printing the plastic and glue a layer of foam to it, cutting away the excess. I'd try PETG, not sure if gluing EVA foam to it would work though.

Length is 256mm, scaling it down a tad in just that dimension might still work for insertion.