The picture just explains it all...sorry I couldn't make an actual full instructable.

Step 1: Materials

-a plastic water bottle
-xacto knife or any other thin sharp cutting tool

Step 2: Cut

Cut it along the first line if your bottle has ridges. for other bottles, it should be about 2.5 inches down starting from the label.

Step 3: Tape and Cap

Take of the cap and tape the edges of the cone-ish shape. It is not necessary to tape the bottom part, but you can.

Step 4: Put It Over

Push the top part, lip first, into the bottom half until it clicks in and can't move. If you look closely, the first picture is not correct, but the second is fit snugly.

Step 5: Put the Device In

Put the device in speakers first in the funnel . The difference might not be very big if the volume is set low but you can hear the difference when you bring it up a few bars. The sound is channeled downwards through the hole and bounces around the walls and bounces out.