Abstract: A penstock comprising a frame having an opening therein and a door slidable within the frame across said opening to close same by vertical movement between fixed vertical guide members slidably engaging the side edge regions of said door, wherein at least one of said members at each side forms a resilient vertical seal, said penstock including a base invert seal which is produced by temporarily and at least partially compressing said resilient vertical seal, forming a pad of resilient material onto which the lower edge of the door in use will close and causing said resilient material to form a seal around the lower end region of the resilient vertical seal prior to releasing the temporary compressive force applied to the latter, the resilient pad being preferably formed by introducing the material in liquid form into a channel defined by limit strips such that the liquid flows around the lower end region of said resilient vertical seal to become bonded thereto, said compressive force being maintained until th