In another configuration, the top ply can include a double wall, and the bottom ply can include {HEAD ~ a single wall} which is received within {TAIL ~ the channel} formed by the top ply double wall 
is received within
In another configuration , the top ply can include a double wall , and the bottom ply can include { HEAD ~ a single wall } which is received within { TAIL ~ the channel } formed by the top ply double wall
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH