You can see the rubber strip under the glass in the below pic. This was part of the fitting kit used to assemble the glass to the guard.
Spend a heap of time with some thinned tectyl and lanolyn working it between the inner and outer pieces. This ensures it won't rust any further.
I'm a little hesitant to using things like mastic as they can trap water as well as prevent it getting in but it may be a good idea on the top of that panel gap as long as it does not trap any existing rust.
Hope this helps,