Hi kaero,
for my application i realized the following solution.
Check the help-file for the vi Append Table to Report VI. It has an input called "column width" and there is the following note:
"If you are using a template and do not want the column width to vary, use the value -1."
I opened the VI and simply changed the default value from the front-panel (sorry, actual program-language is german):

The VI "Append Table to Report" is used in the primary vi "Excel Easy Table". I think the main problem is, that the input "column width" is NOT LED OUT to the primary VI "Excel Easy Table".
I like really like the ReportGenerationToolkit because it's very helpful. But in my opinion this issue is a really weak point...