Got there! The sixth mount was one I needed, saw it immediately this morning.
Usually the preamble states if hidden words are in straight lines, so when I saw mule in a square I thought it must be one… although when the others were in straight lines I had my doubts
I was stuck on the wrong sort of shape for a long time, but when I found the sixth mount I thought that must be one of the chosen ones, and that forced a rethink of the shape
Thanks for your help getting me over the line @quisling