Lifehacker a couple of weeks ago.
Buy replacing the stiff internal feed tube of the spray bottle with flexible hosing and then weighing down the end of the hose with hex nuts, the end of the tube wants to be at the lowest point of the bottle, just like the liquid.
The only issue I have with this hack is that eventually the nuts will rust out and not work any longer. I actually think the rust interacting with your liquid will end up being a larger problem long before the nuts stop weighing down the end.
My suggestion would be to take and weigh down the end with resin or hot glue. I could see making a small mold to put the flexible tubing through and creating a small block of resin or hot glue. Once it has dried you can take the tubing out of the mold and cut the tubing off so the weight would be at the end of the tubing. This would be easier than trying to add the material onto the end of an already cut-to-length piece of tubing.