These are our units, provided by Pedigree Pens LTD, where we house our rescue kitties while they stay with us before finding their forever homes. 
We used to have wooden pens but upgraded to our current uPVC pens, which are so much easier to keep clean! 
Our pens have access to electricity so we can keep our cats warm in the cooler months. We’ve recently installed clear Perspex to the front of the pens too, so that helps keep the wind, rain (and occasional snow!) out and keeps the cats dry and warm.