I just noticed that Home Depot (internet) carries a corrugated HDPE pipe in 6" diameter that is made to interface with D2729 / SDR35 fittings. It is marked at $14.03 per 10 foot section but shipping is additional. It appears to be plastic-lined like the 4" pipe they carry at the stores. This might be a reasonable alternative to SDR35 pipe.
bababrown
bababrown