Since you insist, here's a link for you. See section 3.2.1 for what type of wood medieval builders in the Black Forest area favored. Softwood was generally used, except in the areas where oak-dominated forests abounded. And it says oak was already a lot pricier than pine back then (not hard to believe, as it takes longer to grow, and is harder to cut/trim/work), so many people would use oak only for the visible/decorative parts of the houses.
If softwood is good enough to be chosen by a medieval someone to frame his house, surely it's also fit for a crate, even one that has to hold scary monsters.
P.S. lol at the idea that Douglas Fir grows in the Black Forest.