I don’t know how effectively this would work (and especially when you switch to the mobile menu - it may require two menu versions) but my first thought was to put in a 3-column stack, in the middle have your nav bar, and the left and right have a divider line (either the Foundry built in one or RuleStack from stacks4stacks).
Try putting your parent/nav stack into a CSS Box stack (free from JW)
Use the Class wrapit in the CSS Box stack settings.
Add the below css to the CSS tab of HTML Code
Example at https://casaejardim.pt - scroll down to where it says ‘Trending at C&J’.
This is a header in said CSS Box.
The effect has quite a nice fade out to the edges of the page.