The way I'd do it would be to find a flexible rubber or silicone blanking piece to fit the hole, cut a 20mm diameter hole in the centre, slit the blanking piece to let the wiring loom in, then seal round the loom and the length of the slit with something like Stikaflex sealant (Spelling?), let it set, then insert the blanking piece into the hole in the panel. It should be good for a decade or two, unless it gets too hot in that area.