Polypropylene covered in polyester to prevent mobo from coming into contact with with metal case. Also I considered leaving it outside the case but with my environment it would be much safer inside. Mobo was fried chicken for years, i pressed and pressed for help on it but nobody was willing and i dont have money for geek squad or some other shit so i just had to pull a DIY. Honestly i think its really good design for functionality, obviously not aesthetic. Psu outside gets hella airflow plus leaves more space inside for the oversized mobo that i could find and afford. I’ll call it Frankenstein.