For the dividers, you may want to give them "feet", not sure how to call it. Basically a plate that will encompass the whole bottom of the original bin.
This will help with the divider on "getting a bit out" and the pieces slipping through the opening on the bottom. The other option is to include an assumption the bin will be pre-drilled in the bottom sides area and the dividers to include pressure-notches that will drop in those drilled holes while inserted in the bin.
Either way, for dividers gravity is not enough in practical use.
That said, considering the costs of the original bins, a 50x50 with "integral" divider might be a better option and more economical on filament compared to a pair of 50x25. The same way I am sure a 50x100 with 3 "integral" dividers will be useful.
EDIT: scratch this, with the bins not being separate, it would be a pain to empty them which is a bigger issue the smaller the bin ...
BTW, when I said "1.5 size", I should have mentioned I meant 75x75 for those scenarios where 100x100 is an overkill.
On last nitpick. After seeing the "Streamliner" version on photo it immediately hit me there should be *some* collar included - say 5-10mm high which will have the full-width as a normal Festool bin has.
This so that when the bins are next to each other, the gap between then is not affected by one being a tiny-bit sideways, not sure how to describe it. But if the bin was (from top) first going down vertically, and only then started to taper it would still be as FDM-friendly as possible, have tiny tiny bit more capacity but may form a "matrix" a bit more nicely.
Just a thought.
Is funny in any case for me. I use a Prosperplast NORP injection-molded system - cannot be beat on price/bin as new 20-bin box is $6 or so. It has its limitations but I only now realized one annoyance it has that bugs me is caused but the lacking a collar on the bins. This sometimes causes an "alignment annoyance" where the bins just do not like to "fall" into the position on first try and "rest" themselves on their edges mutually. I suspect the Festools having the top of the bin vertical avoid this.