UPDATED
I have added a bunch of shapes with a flat bottom, these are to place under modular (thin) baseplates on a flat surface, one raises the studs to be on the same level as a plate, one is for two plates, the other takes the studs to brick height. Should be self explanatory, and will allow you to build continuously without the compatibility issues with the older thin plates and the newer roads or regular plates.
These were part of just updating my old designs in to CAD, however if building a city, these done in TPU for grip would be handy for giving your plates a level building surface.
I kept the old files and labelled them, they're more brick-ish, and enabled a similar concept but can be placed on studs.
The author hasn't provided the model origin yet.