I've trawled all the way through the posts and no-one seems to have expressed uncertainty about how to "fill the blank cells to complete a thematic title".
Since the blank cells are not contiguous, it seems that you can fill them from west to east OR from north to south. The latter seems more logical given what is required to complete the thematic title, but either would seem feasible, unless the incorrect choice would have an impacts on the rest of the endgame.
Having found the originators, I'd like to be clear about the above before attempting to tackle the 66 bar conundrum.