Why would you want that enclosure? It was designed for the MK1 Focus, which means there is basically zero guarantee it will work in an MK3.Does this sub enclosure work for a 13?
If you really want an enclosure, I would design you one for free, but all you need is a circular saw, a nail gun of some sort (preferably with finish nails), MDF from home depot, carpet from anywhere you want really and titebond II glue. I built my box for about $50, not including the carpet, but the carpet would make it less than $80 most likely.
If you are down let me know, I have designed quite a few boxes all of which are amazing. If you want to see my work look in my thread called "ST1 Sub install".