After several hours, the silicone seal I'd made to block air flow from going past the oil filler had torn, potentially caused decreased cooling efficiency. So, I tried a layered approach instead. The upper layer is fiberglass reinforced 1/16 inch silicone, which has very little give, and the deeper layer is 1/16 inch "extreme" silicone; both from McMasters. Most of the time replacing the seal is involved in removing and reinstalling the plenum.
