Worzel
Don't know if you've started your build yet but we make pig shelters with 6"x2" timber frames and sheets of 8'x4' sheets of ply screwed together (that way you can take them apart if needed) we use the ply for the roofs too and cover with roofing felt (might add hubby is a builder so we get a lot of this stuff as leftovers from jobs or i'm ultra sneaky and go to the builders merchants and shove it on his account when hes too busy with a big job and won't notice a little extra going thro!!

) The shelters are taller at the front and slope to the back and we lift them into position using the handler and straps (not on a windy day!!!). if you browse the net for field shelters you'll get some good pictures of whats available.
HTH
Mandy

Ps make sure the ground its on is a level as poss to stop wobbles!