Pipe stalls - covering?

(no photos as of yet, sorry)

I'm sure that 99% of you being north of me, you'll find it utterly ridiculous that I either A) don't have suitable stalls set up yet, or B) think it nuts that I'm concerned about winter approaching, but nevertheless, I'm up for a good suggestion or two.

Previous homeowner built two sets of stalls - All out of steel pipe, two have been covered on the sides with plywood & aluminum sheeting, the other two not. All have metal roofing, and the structures are sturdy. That being said, the two that are covered on top & sides have become housing for feed, hay, and lawn care equipment. I would like to cover the sides of another stall to relocate said lawn equipment, and use the covered stall for Chewie's winter retreat. (There were three winter nights & one winter day last season where the poor fellow was greeted by 30F days with blustery 25-30mph winds and high humidity. BRRR! Shiver shiver pony! He was boarded at the time, and enjoyed the retreat of an old vet's barn, with 14x14 stalls, cement bottoms, pipe rail tops, and adorable dutch doors. Stall flooring was nasty, and that's one reason why he doesn't live there anymore.)

The stall I'd like to cover the sides on may eventually be used for horse care (as it's the only one that'd have easy access to one half of the pasture, the smaller piece of it, and I'd be able to close off the gate to keep him restrained for injury or likewise), and the stall that I have in mind for winter retreat needs repairing... What makes a good "side wall covering"? Previous owner has aluminum siding on the outside of the pipes, which is in pretty good condition. Old yucky plywood on the inside of the pipe, some of which has been kicked out or otherwise damaged. I'd like to replace it, but am not sure what to use.

Suggestions? Comments? As a note, please understand I'm not interested in tearing down the pipe-framed structures I have now. Perhaps next year I can tear them down & replace with a nicer facility, once Uncle Sam returns tax monies I've loaned him this year, or my employer dumps the magic large bonus I dream of ..

TIA