Over the years I've made up quite a few of my own using silicone (or similar) and cling film. Spread a piece of cling film across opening to be gasketted and the piece to be coupled to it, squirt on a continuous bead of silicone and then place the parts together gently. Insert screws through holes and align. Screw up to within 1.5 to 2mm. Let the silicone cure. Once cured, peel off cling film (or leave on, doesn't really matter) and cut off excess cured silicone. Reassemble and screw down tightly. Job done.
Rubber pond liner is what I use, comes in a variety of thicknesses and by the meter. I source it from a local hardware shop (Cambridge) so can't point you to anyone in your neck of the woods but it's available online.