Template part in iFrame

Hey guys,

(As a preface, I know this is going to sound absurd, but please bear with me. I’m aware this is unideal and atypical.)

What I need to do is put wp_nav_menu inside an iFrame so I can view it on a non-WordPress-powered site. (Essentially, I have a WP-powered site where the user will have to leave the site to make a purchase. However, on that site, I’d like the page to look, feel and function — primarily from the backend — in the same way the rest of the WP-powered site would.)

I would create a separate nav.php and try to embed that in an iFrame, but WP blocks direct access to a file in the theme folder. Is there somewhere else I could put it? Or is there some other way I can unblock access to this file? (Not the route I’d prefer to go, but if it works, I’ll take it)

My only other option is using something like a nav.html page, but I have no idea how I can run the usual WP PHP functions within this .html page.

Any and all help will be much appreciated. Thanks in advance.