From memory I think the instructions tell you to glue the bellmouth onto the pipe with silicone; I don't do this but it does need to be secured. I pop riveted my bellmouth to the pipe and didn't bother with any silicone for sealing. Drilling it is ok with a sharp bit but make sure you support the pipe on the opposite side to where you are drilling to stop the pipe collapsing. I've yet to fly mine but this is the same method I used on my Comp ARF Flash which worked fine.