Good trouble shooting Chin!
This is why one should test things out before permanently sealing them. In my case I had a plan but lacked the parts (I was in a big fat hurry). I will insert a 1 1/2" ABS threaded coupler before the elbow so that I can remove the skimmate drain and use a bottle washing brush inside. I imagine your solution would work too, especially if you could place an O ring on the neck to prevent the skimmate from seeping out between the neck and the ABS elbow.
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