IMO you'd be better off gluing a thin liner for the interior of the enclosure, as completely removing urate and feces from the OSB grain isn't going to work very well. Then just seal the corners/edges with the silicone. Think something like this:
http://www.homedepot.com/p/1-16-in-x...3003/202090190
Use the chemical sealer on the outside to protect against the occasional water spill.