It's a EPDM liner it's rubber made by Firestone. The plastic ones break down over time in the sun. Also you can buy EPDM in any length and width. Got mine on a huge roll should of seen the comedy of me getting it in the back yard. Also was before I had my big truck borrowed a buddies little Nissan pickup hit the brakes it pitched sideways when the roll shifted from side to side and sent me in all types of directions. It was definitely a Sphincter clinching moment.

 @USMC: jesus ajajaja those loads are scary

ahh yeah I used one thick plastic used at constructions sites, but I doubt I can get my hands in one of those EPDM

 @its-chris: 2 fish in there are over 20 inches long can't see them in this picture

 @USMC: I discovered I can get that liner here!

well I will be looking for some soon.

daaamn those are big! mines are pretty small at the moment

 @its-chris: If you get it get enough for some scrap. I keep it around for a patch kit if I need it. Been 9 years now with no issue but better to have and not need.

 @USMC: read about it being repairable, it might end being cheaper totally will look into it