Also, you don't want so much of the extension to be honey combed that it falls apart. (Something about structural integrity, though I have no idea what integrity means.)

I'd drill a whole bunch of little holes (in my spare time -har-har-har). If the pressure at any point was still too strong, dry the extension tube out with a paper towel and let it sun dry a little while. (Almost said a little bit.) Then drill with a slightly larger bit in those holes.

Its always easier to expand small holes to larger holes than vice versa. ;)

Hope the rest of you lurking don't mind "this drill." I realize, it could be boring. ;)