So I've kind of "mushed" together a base that is working fine with my harbor freight medium storage cases. It fits snugly, aligns correctly, uses the small right/left pockets.
I used kennetek's openSCAD scripts to create the bases, mashed them together in cura so the walls are butted up against each other, sliced, printed, and after 22.5 hours, voila, a nice one-piece base. Check it out at kennetek/gridfinity-rebuilt-openscad: A ground-up rebuild of the stock gridfinity bins in OpenSCAD (github.com)
It fits snugly, and if needed, I think a couple short 2.5mm flat head screws could be driven through the back/front/side or even the bottom to hold it in place, or a touch of glue, although I'm not exactly sure what plastic the case is made from, so glue might be tough if it's a LE plastic.
The bins I'm putting in, don't have a lip, and the dimples on the top don't seem to be an issue.
It has a couple defects, that I will resolve, no dealbreakers, but the rounded corners where the 2 pieces meet looks odd. I could fix it in fusion or something, but that was another step that I think makes it more complicated for people. Eventually I intend to tweak the openscad script, or use the cadquery version to just generate it directly.
I did make both the minimal base, and the skeletonized version here, they both fit fine.
The changes to the openSCAD script are entirely in variable definitions.
One caveat is that I use a kobra max printer, so I can print the whole thing at once. If you don't have a large format printer, you may have to print half at a time, using the "screw together" style_plat, and bolt them together and then drop them in, I have not tested it, but I have run the script to see what's generated, and it appears dimensionally correct.
This one in the pic is printed in PETG, which I happened to be wanting to get rid of, normally, PLA+
The attached STL files were generated with the following parameters, which you can tweak a tad if you need to.
Parameters you need: (I'm using the kennetek scripts from github dated 2023-03-12.)
For the large baseplate:
For the small baseplate
I use cura as my slicer, and so here's what my layout looks like:
place the small one butted up against the big plate, and even with the left edge. Then add the small model again, use the mirror function to mirror it right-to-left, then align the newly-added one against the model, butted up against the bottom edge, but lined up against the right.
I used 3 walls, 2 top/bottom layers, 15% infill. Bases don't have to be super strong, but YMMV.
I intend to make something for the large box as well.
The author remixed this model.