Extoil hasn't had ("sold out") caps available for their oil extractors due to “unexpected demand” and doesn't seem to be getting anymore in. I designed a rudimentary plug to fill the top opening on their oil extractor to meet my needs. There is an allowance for a o-ring in the design, though I haven't sourced and fit one yet. There is room for customization and improvement. May or may not meet your needs.
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The author marked this model as their own original creation.