This is the deck that goes with the Xtracycle SideCar. A deckless SideCar is a sad thing.
The 100% post-consumer HDPE deck is manufactured in Minnesota from recycled municipal garbage cans. (HPDE is what your plastic cutting board is likely made of, and it makes a great deck for the same reasons: it is waterproof, UV proof, resists scratches and dings, and is very strong.)
Hardware is not included; use the hardware included on the SideCar. If you’ve lost that hardware, use M4 bolts (like the ones used on your water bottle cages), just make sure they are at least 15mm long.