What would be the best "substrate" for these Kratky bottle plugs to hold seeds or cuttings in place?

I designed and printed these adapters, which fit into used glass bottles. I use them to grow lettuce, basil, and more.
I will publish the files very soon and also make a post about them :)

Right now, I'm trying different materials to use as substrate for holding the seeds, but most I tried sucked in one way or another.

• LECA is too coarse, and the balls roll out.
• Coco coir is washed out quickly when filling the bottles/ when raining, and rots in the nutrient solution
• Polyester filling (used for pillows) is either completely soaked, or bone dry, but works reasonably well
• PVA sponges become too hard when dry
• Cotton/ other cellulose stuff (towel, etc.) decompose too quickly

Can you suggest me other materials that would be best suited for that purpose?