When scanning objects like sculptures using Kiri Engine (photogrammetry) often the bottom is open. What I currently do after exporting to OBJ, is opening it in my RevoScan app and use the "fill holes" feature to select it and fill it. However that's usually not entirely flat and suitable for printing and results in artifacts. What I then do it simply lower the object when slicing to sub-zero/a negative value to have a small part of the object be under the bed, basically cutting it off and leaving a perfectly flat surface in my gcode.
However, I want to really be able to do this in my STL/OBJ file already for publishing to Thingiverse. How do I alter the STL/OBJ file to basically achieve the same, but in my object rather than in the final gcode?