Do You Mean Chopped Insulation?
Speaker Cabinet Batting Would Work.
Easy to Install & Remove and No Itchy Fiberglass.
Try Pipe Insulation Tape on the Sides and Bottom of the Gullies.
https://www.mcmaster.com/#condensation-reducing-tape/=17v9nso
Or
At Your Local Hardware Store.
Put Sound Dampening Material on the Walls and Bottom. $$$
https://www.mcmaster.com/#sound-absorbing-foam/=17v9tbq
Use Double Side Carpet Tape to Hold the Panels in Place.
I Would Try the Sides and Inside Bottom First to Keep the Mess and Expense Down.
Doing the Gullies are Properly Not Worth Doing.