I wonder if anyone here knows what this thing is for?
Typically engine intakes have a certain resonance frequency. At this frequency you will lose power and gain noise. These little intake bags effectively remove that frequency so you don't have then power drop and don't have the resonant noise at that RPM.
I doubt you will notice the power loss - so it can't hurt - as long as your "fix" lasts the life of the car. If ANYTHING gets into the intake tract unfiltered in will likely wreak havoc with your engine. Using silicon glue and plugging from the inside sounds like the safest method to me. You guys that used epoxy and said everything has been fine for a year - well good luck. I hope your epoxy handles the vibration and heat of an engine compartment for 3+ years. If BMW sells a part like this without the extra air tank, that would be the way to go.
Good luck, and enjoy!