I put one of those in my P5 about 4 years ago. I was a little concerned that the heat from the radiator might melt it, but no problems. Got mine at Lowes.
Have you fastened it down at all or is it just sitting there?
Edit: Thanks for the idea guys. Found a 12 qt Rubbermaid in the house. Formed a relief for the rad overflow hose and mounted container to one side so overflow bottle can still be accessed. Also tie wrapped in place so won’t move and then filled holes with some gasket maker. Tie wraps can wear on metal, definitely take the paint off, so I use these new grip lock tie wraps that have a rubber bead bonded to inside, they are also removable.