Welping box/ pup run

[1whitedog]

Looking to build a 8 by 4 foot run with some kind of crated floor and wire sided. Attached to the back a 4x4 foot dog box. Place for gype to raise pups then let them in until about 3 months. Any do or don'ts from those of you who have pup pens. What materials would you use or stay away from. What material for bedding. My idea is all metal to sanitize between litters.