The typical temperature envirnoment in a car tends to be on the warm side (even in a shaded place like the trunk) sun/heat builds in confined spaces and temps can often exceed 160F during the summer heat. HDPE and LDPE, (High density Polypropylene and Low Densisity Polypropylene) have some of the lowest melt temperatures of any plastic, and to further that, it becomes unstable and starts to warp/sag at even lower temps. It's also one of the most dimensionally unstable plastics out there having little "backbone" or rigidity meaning that large flat unsupported areas of a molded part are likely to sag or warp with very little weight applied to them or when subjected to only moderate heat.

Bottom line? If you want a dimensionally stable long term fuel cell, get an aluminum one (I'd get with a good liner to prevent any future).