How do I display the US flag at half staff with a standard house flag pole holder?
I couldn't find a more 'official' site that lists this custom, but flying a black streamer with the flag is appropriate when the flag cannot physically be lowered.  This blog posting is one random source, but if you search for &quot;half mast black streamer&quot; you get many results.

Possibly a more reputable source (worthiness left for you to decide):
http://www.cnn.com/2011/LIVING/05/30/fly.american.flag.mf/
Also: http://eventresources.com/wp-content/uploads/2018/06/How-to-Properly-Hang-the-American-Flag.pdf

What if I can't fly my flag at half-staff?
Some flags, like the ones commonly seen in school classrooms or on
houses, are fixed in a certain position on their poles. How does one
handle the sticky situation of a flag that physically can't be flown
at half-staff? The United States Code doesn't cover this conundrum,
but the American Legion advocates adding a black ribbon to the top of
the flag's pole to indicate mourning.