I was looking for files like this, and most of the files didn't have just a plain tray. Here is a plain tray for erasers, spray bottles, and/or markers. So, I developed a tray that didn't use a ton of filament and was sturdy enough to hold stuff. On the back, there are 2 slots for strong magnets, and I used circles that were 1.25 inches in diameter.
NOTE: I have 2 different files. The first one is for printers that can print one layer with lots of filament without breaks. The second one is for printers like mine that cannot handle a ton of filament at once.
I love printing. In my free time, I am almost always 3D printing something or designing. I use my Original Prusa i3 MK3S (with a semi-reliable MMU2S) to print all my models (well, all models after September 2018) and teach some kids at the local Elementary School. :)