Wrapping fibreglass/ rock wool to seal in fibres in acoustic panels (bass traps)
You would do fine using simple burlap. Given the wavelengths of the signal that must pass through, cotton fibers will pose no problem. They are also considerably smaller than the coarse fibers found in commercial acoustic panels. In fact, any material used as speaker cover would work for your application. Incidentally, fiberglass is a very hard and acoustically very reflective material. You may do better with a panel of dense open cell, soft urethane-based foam; they may be found at a local craft store.