I mounted the bottle on the plastic panel that is right above where the spare tire is but the panel along with the bottle doesnt stay in place whenever I go over a bump. Is there anyway to keep the panel in place?

 

Have you try finding a different mounting place in the trunk? some where that has a solid ground? Or you could go with a jig saw and get a thin piece of ply wood and trim that piece of wood to the exact shape of the plastic piece, then it should be a little more sturdy.

-Tu-

 

You mounted it on a thin piece of plastic and expected it to stay? You should remount the bottle on something more sturdy, like metal. The plastic will crack/break sooner or later, so why not save yourself the time and money?