If you handle the figures with care I doubt you´d really be damaging them with those poly bags, I use these as well also for shipping but for extra protection you can wrap them in bubble wrap. Especially when you place multiples on top of each other I´d advice to place a bubble wrap layer or something similar in between each layer of figures. For (long time) storage the most important thing to keep in mind is a certain consistency in temperature and humidity because that´s something that can really affect the material the Beasts are made of. Avoiding extremes in both temperature and humidity is crucial as well as rapidly changing conditions. So not only the way you pack them up is important but also the place where you store them. I assume your plastic storage boxes are not transparent allowing any light to come in because if you store them it´d be best to store them in the dark.
There´s not a lot known about long term storage of these figures but we obviously know from the past that when Beasts were discovered after many years and were taken out of storage the condition in which they´ve shown up really varies a lot. I´ve seen Beasts in a completely faded and dried out condition coming straight out of storage, these have probably been exposed to extremely high temperature with a very low humidity causing the base material to completely dry out also affecting the colors. But there are luckily also enough examples of Beasts that show up almost pristine after being taken out of storage decades later simply because they´re kept in better conditions.
All the Beasts I currently don´t have on display and which are in boxes are in a dark closet in which there´s not a whole lot of extreme temperature change and I check these regularly to see how they´re holding up.
To obtain a more stable climate it might also be worth considering to place them inside a EPS box this would instantly create a way more stable condition and you´ll hardly have to worry about extreme shifts in temperature or humidity.
You can make it as extreme as you want with packaging but in general the Beasts were made to last as they´ve proven to be holding up pretty well over time so they´ll probably also survive a couple more years inside a storage box if you avoid any extremes.
Enjoy the Beasts!
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