They make a two-part material for making moldable in ear hearing protection that has neutral sound. Harder material, wood as an example really bumps the upper mids and highs.
Open cell foam is lousy, as @noromance said.
A wide pillow would work as @oldhvymec said if your head rests on it, with a semi reflective surface applied to the sides, something as minimal as paper. I use a memory foam pillow behind my head , since my system is set up in the room width wise, that really helps with the acoustics since the rear wall is about 16-18” behind my head.
I use "Clicks" reading glasses, and just taped a piece of cardboard to each side behind my ears angled in about 30°. That works well, so I now have an unobtrusive way to mount something that I can do some experimenting with, and the added benefit of already being accustomed to them being there. Does this make them reading-listening glasses?