My "kids" think the best tunnel type play thing for them is the 4" in
diameter, corrugated style, black "sewer or drain" pipe from the local
discount do-it-yourself hardware outlet.  The pipe sells for $2.10 per ten
foot piece and is easily cut with a utility knife and it is flexible with
ridges inside for climbing.  It is even "vented" with air slots from end to
end.  "T" s, and "L"s are available, though they cost a little more.  I
refuse to pay for those habitrail-type things when this works as well or
better, and for far less money.
 
Three and four foot long pieces work just fine in my ferret "playpens", and
they have hours of fun chasing each other through them, and trying to
out-smart each other.
[Posted in FML issue 1463]