by the hood 5 and demarcated by base 2 the faces 11 of hoods 5 perpendicular to longitudinal direction 3 are smooth and are accordingly positioned directly next to each other or one behind another without being covered in the vicinity of at least one of the two faces 11 each hood 5 has a u shaped seal 12 fig2 inserted in a groove hoods 5 are compressed or forced together in longitudinal direction 3 by unillustrated tensioning mechanism ensuring the requisite seal between the hoods the embodiment illustrated in fig1 through 4 has in addition to track 9 which is not extensive enough vertically to function as a hinge a take apart hinge structure on each side of tunnel 1 specifically in the vicinity of both walls 7 or free edges 8 of hood 5 these hinges 13 are local and each consists of a rib like elevation 14 in the form of a block of plastic that as will be evident from fig3 is short in longitudinal direction 3 and of a groove like depression 15 in the free edge 8 of hood 5 each hood 5 accordingly has four such hinges 13 fig2 is a section through a hinge 13 it will also be evident that the hinge extends considerably farther vertically than track 9 does depressions 15 are also longer in longitudinal direction 3 than elevations 14 are ensuring the requisite play in longitudinal direction 3 in the vicinity of each wall 7 is an outward extension 16 that covers the side of base 2.