A treated 4x4 would be good for the corners... and I don't think the feed store has those... but Ace does. Get a bag of fairly hurky nails too.
If we cut a 4x4 and made the box corner by nailing the boards to it, that'd work nicely, and, theoretically if we don't fasten it to the ground, it ~could be~ moved later if you decide it's in the wrong place.
Doing the math, if you use a 12 inch section for each corner, one 8 foot post would give you eight corners. Then we can look and see if it needs stiffening at the halfway point... with something on the outside - you know.. if you have more of these boards than you know what to do with, we could just double them up to make it a little stronger.
This is all assuming those boards aren't already gone...or that they don't insist on their full price, which'd be seventy bucks or so for just the six footers.
I guess worst case...I'll offer to take 32, which would be enough to double up on two beds, *or* give me a bunch left over to use. That'd be forty, and I imagine I could talk 'em down to thirty or thirty five.