From what I've heard the biggest culprits on throwing aerosols in brass are trumpet in the high range and trombone because of the lack of bends in the horn.
I cut the top pocket and use it to wrap around the bell with the back open and just use a rubber band to hold the top of the cover on the barrel.
Total cost was about $40 for the high school band. I also made an air deflector to wrap around the flute headjoint and aim the air down towards the players lap. $15 for laminate pouches that I cut 4" tall each. 3/4" hole on each side to place the headjoint through.
I'm just looking to do all I can to protect the students and give parents some peace of mind.