I sealed mine in with butyl tape, and it seems to be a decent fix.
except for the round insert lens for the back-up light. That one doesn't have a lot of sealing surface area.
As far as a gasket, I'd say thickness isn't all thagt critical, but it needs to have long lasting elastic properties, UV resistance
and I figure you really need a gasket under the black backer plate as well as under the lens itself.
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