Man, following you on these threads is re-living some memories I'd soon forget. Don't get me wrong, I love animals, but - horses - argh!
Money pit.
For your temp shelter, I will not comment on use, or what is best....
Go to your local lumber company.
Buy some 4x4 pressure treated posts, T-111, and lots of 2X4's. Build everything to the dimension of the T-111 4X8 sheet. Go twelve feet high in the back and sixteen high in the front. For a roof, here is where the money is. Get a good metal material.
I have built six or seven, I lost count of these type of run-in's over the years. Simple, inexpensice and easy to deconstruct. I actually moved one, the rest, last I checked were still standing.
Anywhere near the ground, I used pressure treated wood. A post-hole digger, hammer, nails, tape-measure, saw and level was all I used. Built them right out in the field. Mostly, me, myself and I. For the run-in, I went 16' deep and 24' wide.
I'll look for some construction pics when I get back to the farm tonight.