If you add a "not found" layout you can add modules to it.
1) add a new layout and call it "not found" or whatever you like
2) set the route to error/not_found
3) add your modules
now you can tease the customers in (hopefully) not leaving so fast.
be 'implanted' into the Theme Error File directly, to avoid additional work.
Just an idea...
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Back in the day (oscommerce) I used the 404 page to suggest similar products to the one they were looking for.
so simple, add your current modules as is or write specific modules for the purpose.