Thanks for the answers!

I tried 5% with PolyFlex, grid pattern. It's very solid still. Buttons are quite small, only 1-2 cm o. So I switched to 0% infill, but it's still very solid, because of the walls.
I'm using a Trinus and I don't want to use any material that needs a special extruder.

Which one is softer, NinjaFlex or FilaFlex?