Can't say I ever had - but I see where you are coming from - Wood may not be a good alternative afterall..

Finding square sided non DWV pipe is difficult and when found questionable - the one example I found did not have any marking indicating where it was made or certifications (ie NSF-61 or NSF-Pw -- water systems safe)

http://www.amazon.com/PVC-Hollow-Rectangular-Gray-NSF-61/dp/B003Y39WD6

6" water safe round PVC pipe are somewhat easy to find - downside is the difficulty to cut and install 45 degree pods/wyes into it - the flat sides are needed to make the process much easier...