Full container width will work.
My mega menu is set to full, but the menu is pushed to the left and 1/3 is cut off.
Look at: https://starbasevisitors.com/
Hove the mouse over "Products"
I have installed finion template on a "test" site because the mega menu behaves as needed. See: http://starbasebusiness.com/index.php/en/
Then hover over "Pages"
In that template the mega menu is set to full and there is a custom class of: custom-full-menu You would think there would be a custom.css file - none.
I cannot find where that is in the css.
???