Straw might be useful if you have no other materials around, but the natural material for hogs to use is leaves. Lots and lots of leaves! They prefer medium sized leaves. They will also use long ornamental grasses to weave the leaves together. Inevitably, in a good hibernation nest, far more material will be used that you expect. Leaves are layed flat against each other like tiles, intersecting each other and many layers of them, to make the nest waterproof. It seems amazing that hogs can create such a structure, but they do. They need to be able to make their nest waterproof because they may not always be building it in a box/igloo.
So if anyone has access to leaves, I would provide the hog a good source of those nearby. If you have long, ornamental grasses in your garden, then maybe cut some of that and leave little piles around as well.
