It would be quite awkward to deal with a checksum or magic number for any single file. This had better to be done resource-wise.

The commercial edition does not support resources, thus you need to implement file protection yourself. The file size won't do because a hacker could easily use a model with the same size. The best ways would be either using the add_buffer() function for implementing an own resource reader, or using some checksum algorithm that runs over all files in your folder.