Not sure if this is common knowledge in the community but I discovered today that…
1/4" thick plywood cut down to 11 1/4" x 15 1/4" sheets fits perfectly into the slots intended for the drawers in the 10 Drawer Rolling Cart by Simply Tidy (currently on sale for $29 at Michaels, regularly $49).
Without any other modifications, this makes 9 pull out shelves that can each hold a 6x9 Gridfinity grid, providing 486 grid squares (54 per shelf) in a very compact space.
A few minor modifications to move the two top cross braces should allow adding a 10th shelf, for a grand total of 540 grid squares.
Caveats:
You’ll probably want to use a base plate that can be screwed down, because there are no walls on these shelves to keep the base plate in position.
The frame is too flimsy to store anything very heavy (e.g. socket sets, 1-2-3 blocks, crowbars, etc), but it looks like it’ll be great for holding tons of random nuts, bots, screws, assorted usb cables, jewelry making supplies, microscope slides, and other doodads and whatnots in an very compact space for super cheap (as long as you have the means to cut the plywood precisely).
You could skip the plywood and just use the drawers that it comes with, but the sloping sides of the drawers severely impacts the space available for the grid, allowing for only a 5x7 grid (with large unused gaps around the sides) with 35 grid squares per drawer.
But, maybe that’s plenty for your purpose, in which case, rock on!