As far as bell covers, these are from the FAQ's from nfhs.org
What blend of polyester and spandex is needed for bell covers?
Answer: As this study has progressed the recommendation now stands at any material as long as there is a MERV 13 filter material lining the bell cover.
Does this study recommend certain bell covers?
Answer: No, bell covers should be well fitting and lined with a MERV 13 filter material.
Some are suggesting bags or covers for open-holed wind instruments like the clarinet, but those would make it nearly impossible to teach a young student. Are there any suggestions for further mitigation for younger clarinet players?
Answer: The bags around instruments is also not a current mitigation suggestion as more study is needed. The current bag models that were shown in the study was used for collecting aerosols. However, there are a lot of practical issues with this type of mitigation strategies and more study is still needed to determine its efficacy.
Also from nfhs:
Ideally, bell covers should be made of non-stretchy material that has a Minimum Efficiency Reporting Value (MERV) of 13 – a rating known to protect against cough and sneeze, bacteria and virus particles. However, any type of covering is better than nothing.