In addition to the dryer tubes what I find very good and not too expensive
is 4 inch pvc sewer pipe which can be obtained at hardware stores home base
and similar type stores.  I also find pieces at the flea markets as well as
the couplings, "T's" , elbows and other pieces.  I terminate these subways
in boxes, elevate one end on one tube slightly to provide for hill climbs.
Ow well enough thought i might add this to the dryer tube users inventory.
I thank u as well as our 14 critters.
[Posted in FML issue 1895]