I use a plastic flask. More expensive than a soda bottle, certainly not essential, but I'm happier with a more durable plastic and flat shape for packing that this offers. I did need to replace the O ring once after a few years of having it.
Also with this one, the cap is a measured shot (1.5 oz) so you can get an idea of how much to use consistently, etc.
Here's a recent photo of it with a banana slug in the Santa Cruz mountains.
