I like Pax74's cardboard corner brackets, but I also needed a shape that didn't exist yet to be the middle corner of a set of drawers - so it needed to be the 3 way angle piece plus another angle below to hold things together.
While I was at it, I thought I'd upload the shapes as individual files, because the original version comes into Cura with every shape in there, but I don't need every one all the time. So breaking them out makes them easier to import and place just the number needed.
ETA: I realized I could use an inner 5-way corner and put it together, along with the platform-base variants of both and the capped-bar I forgot to upload yesterday. I'm not entirely sure how to cope with the thickness needed by the divider in various spots, but it should work for holding cardboard generally together as long as it doesn't need to be perfectly flush in all the directions and having the gap of the divider is OK.