is it safe to store the liquid in plastic or is glass essential?

not sur i follow your question

if you are basing the mhrb with lye you can use glass or HDPE plastic(hdpe is the only type of plastic to use if you are using lye) (what abom uses I've heard). HDPE plastic is the type used for milk jugs and vinegar bottles (abom uses the vinegar bottles - can do 200gs of material easily in a 1 gallon vinegar bottle- they are thicker and do not deform as much - also plastic is more forgiving if you bump it into something or drop it.)

if you are using another base (lime or carbonate) you can use either as well, but the danger level is much lower. if you are doing smaller batches like 30-50gs of mhrb you can use a canning jar (or when you buy spaghetti, get the ones that use canning jars with the tops - they work perfect for small batches or for mixing up your lye with water and waiting for it to cool and dropping it in your final base container)