I have another concern about the sod for your ferret pen.  The bottom
layer of sod is a plastic mesh.  This mesh is like heavy fishing line
woven into squares.  If (when) the ferret digs, it could become
entangled.  A ferret might look at it as something to eat.  A friend lost
a wee one years ago after he ate beading string.  I have adopted Desert
Tortoises.  Their yard area is half sod.  Last summer one female decided
to dig the sod and got the mesh tangled around her legs.  I expect a
ferret could run into some problems too, I know you will want to watch
closely.  Your play area sounds just wonderful.
 
Ardith
[Posted in FML issue 4916]