[MrFlippant]

Harbor Freight Tool Box Front Panel Filler for Slot Car Box Working Surface

https://www.printables.com/model/1364391


   

[MrFlippant]

I've also uploaded a variation with the sides trimmed off so that it will fit on popular printers such as Prusa i3, Ender 3, Bambu P1/A1/X1 and the like, as well as a version without the channels for use as a bench/table top work surface with parts pockets and a guide slot.