1. Take off silencer.

2. Turn over

3. Cut 3 inch piece off sealed end

Voila, perfect fit. At least if I remember correctly.

 

Or you could do it the way I did it and use wood. Get yourself a nice 4 x 4 piece of scrap wood that's just laying around, measure it out and cut it to fit. Then, take some thick plastic (I used a think sandwich bag with the zip top), wrap it around the wood and then staple it down to the side that is going to be facing the bottom. Sealed and smooth. Put it in there, put the metal clamp back on and your done. It's a lot heavier than hollow plastic bottles, so it's very unlikely it could be sucked up. Haven't had any problems with mine in the year and a half I've had it in (just make sure you use the plastic to make sure it sealed tight).