I have to admit, while I am enjoying reading all of this discussion of wood and theoretical construction, I am very much puzzled as to why the focus has been so much on real world physics, based on real world history. D&D is set and flavored around a certain time frame in real world history, but it differs dramatically from that time frame because of magic. There is magic armor, magic weapons, and magic items that all operate differently than their real life counterparts would. I feel that assuming there is some kind of magic involved with presumably experienced hunters capturing a high level monster makes sense, and that they would have the resources to own a more expensive, potentially lightweight yet sturdy cage than novice hunters would makes sense as well.