a new version:
Not very big change comparing to the previous version, but with 1) bigger holes on the center in case we need some thick cable, and 2) overall thinner structure, which come lighter and cheaper and still fits the desired dimensions.
On the attached picture comes with the schematic of the cap and the inner mold. The outer mold is the same as  the previous design and can be reused.

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(The molding parts for the plastics cap, which should be sufficient enough to make the final product)

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(The second design, which most conditions have been considered)
--The center hole is for the HV/signal cable to come out, while the two smaller holes beside it will allow a cable-tie to stablize the cables.

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(The old protoype, which is a discarded idea --- because several factors are not taking into accounts)