How do I search for a mixed smart-normal switch plate?
The large rectangular opening is called &quot;Decora&quot;
There are only 3 popular shapes:

Small slot:  the traditional switch, like you have now.
Two ovals: for traditional receptacles (also dual switches)
Large rectangle aka &quot;Decora&quot;: which covers your smart switch, AND all Decora switches, AND all Decora receptacles, including all GFCI and AFCI receptacles and switches.


The big three. &quot;blank&quot; not pictured.  There are others, but they're rare.
Any competent hardware store will stock 2-gang cover plates of every possible combination, including &quot;1 side blank&quot; and &quot;both sides blank&quot;.  Typically in at least 3 colors!

They also make plain switches in Decora, so if all else fails, you could replace the other switch with a &quot;Decora plain switch&quot;, wired exactly the same.  And then you could use a 2-gang &quot;Decora both sides&quot; cover plate.  Those are so common Walmart stocks them.

graphic courtesy Kyle Switch Plates
source of graphic