We use a re-purposed hay wagon as a portable pig shelter. I can store lots of stuff on top of the wagon: a junk freezer with a good lid provides feed storage, a 300 gallon tank holds whey. The wagon provides shade for a dozen medium pigs. When they get larger I will attach the 'awning' -basically a framed out 4x8 tarp. This works really well for our basically flat farm fields, but might not be a good match for a woods situation. I move my pigs every 3-5 days to a fresh patch of pasture. We see planting annuals for our pigs ( peas and oats, turnips and kale). Perhaps too small of an area, but it was really hard on the land and we have since switched to a permanent pasture mix heavy in alfalfa and clovers. Just hand overseeding the spots they root up.