You’re looking at something that’s dense, a bit squishy, but takes effort to compress. That puts it ptobably in the category of either XLPE or EVA foam.

Cross-Linked Polyethylene (XLPE)

Smooth outside, super fine closed-cell texture
Somewhat difficult to compress, bounces back fast
Used in stuff like Pelican case inserts, insulation, flotation pads
Basically waterproof, holds up well to abuse

EVA Foam

Slightly softer and more flexible than XLPE
Still dense, but easier to push in
Shows up in gym mats, cosplay armor, cheap padding, shoe soles
More likely to soak up a little moisture if submerged

How to Tell Them Apart


Thing to Check	XLPE	EVA
Texture	Finer, more uniform	A bit more irregular
Surface Feel	Smooth, maybe a little glossy	Matte, slight texture
Water Resistance	Almost totally waterproof	Mostly, but not quite as good
Squish Factor	Firm, takes effort	Softer, more give
Nail Press Test	Barely leaves a mark	Leaves a temporary dent

Quick DIY Tests

Water dunk: If it stays dry, probably XLPE. If it soaks even a little, probably EVA.
Fingernail press: XLPE resists. EVA gives in for a second.

If you know what this piece came from (like a shipping insert, exercise mat, insulation panel) that can help narrow it down even more. Let me know and I will give it a shot.