I wasn’t thinking of seal + silicone but just silicone. I’m thinking that placing a cling film over the opening of the fridge and place a couple of spacers to keep the lid slightly proud of the work surface, or place a solid object under the lid so that when the lid is closed it won’t push out all the silicone. Apply a generous bead and allow to dry. Trim excess with a sharp blade.
Having said all that, my 2005 B39c fridge lid never had a gasket and I’m pretty sure there is no insulation on the inside of the lid. Med summer ambient in the cabin is easily 40C, the fridge suffers a little but filling up the space with beer cans, mineral water bottles etc seems to help retain a low temperature