I bought a premade one of these for my camper but it was too big, so I decided to make my own and post the design. I am very pleased with how well it turned out. This has been designed to fit on some of the smaller printers beds (aka Prusa) and still hold 10~12 grocery bags. The plastic inserts are optional, however adding them makes it look much more like a finished product. Mine in the photos was made in ABS and then vapor smoothed.
1) Print all parts
2) Sand and test fit each piece
3) Glue together