go down to your local auto parts store.. buy a tube of "Ultra Black"
disassemble the unit entirely and make sure it is spotless clean (any oil reside will probably foul the repair, soap and water are fine, there are brakecleans that are fine but you need to know which is which.. stick to the soap and water)
on the housing put a nice bead of the Ultra Black on the housing, lay the lexan down, and then the cover.. put a small amount of the Ultra black on the screw threads as well..
assemble and allow to set for 24 hours.. should be just fine..
DO NOT USE regular gaset silicone, the stuff is destroyed by oil.. The ultra black is a different compound and will stand up to the oil.. (but will melt if not allowed to fully cure before use)
if you give up, ship me the cover and I will fix it for you..