I built it all from scratch (pun intended).
Post pics of walk-in run covered in hardware cloth - Page 5
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The corners of the run are 2x4's (that come together with adjoining wall 2 x 4's). The walls are framed just as you would frame any wall. The walls are sitting on a sill, which has a leveled gravel base. Very easy to do and has held up well. As of today I have yet to make any repairs to anything.