What about something like these:
https://www.amazon.com/Strands-Natural-Crystal-Gemstone-Jewelry/dp/B07TW7ZQCX/ref=sr_1_3?crid=88OF6ELIUMID&dchild=1&keywords=4mm+quartz+beads&qid=1587014623&s=beauty&sprefix=4mm+quartz+beads,beauty,187&sr=1-3-catcorr&th=1
Genuine quartz beads and the hole in the middle might improve airflow.
https://www.amazon.com/Strands-Natural-Crystal-Gemstone-Jewelry/dp/B07TW7ZQCX/ref=sr_1_3?crid=88OF6ELIUMID&dchild=1&keywords=4mm+quartz+beads&qid=1587014623&s=beauty&sprefix=4mm+quartz+beads,beauty,187&sr=1-3-catcorr&th=1
Genuine quartz beads and the hole in the middle might improve airflow.