Make links open in an iFrame

I am trying to build a simple site to quickly go through several news website sources using iframes with a side menu on the left column with links pointing to an iframe on the right 85% of the screen.

I am looking for a way that this external link plugin could map any links that I don’t specifically have set to target=”_blank” in the wordpress menu to open in a specific iframe. In the settings I see an option to open links “in topmost frame” which seems close to what I need but does not work.

Right now my work around is creating a menu on a test page, viewing HTML, finding and replacing target=”_blank” with target=”iframe_test” then pasting the menu code into an html block on the actual website homepage.

Is this possible?

The page I need help with: [log in to see the link]