JR, the container you show is most likely a polystyrene (look for a "6" in the recycle symbol) and more brittle than what you need. Look for a container or (as Zath indicated) a "blister" type packaging for a suitable plastic. Plastics are best 'thermoformed' meaning affix it to a form and apply an appropriate amount of heat to 'set' the shape. Cubs are pretty simple re: the glazing with only 2D curvatures to deal with.
You have had luck with experimenting with your materials, so I know you'll "lick the problem".
