How wide for surface skimmer box?

fishgate

Active member
I am drilling and installing a surface skimmer box for my 120 gallon. How wide generally does the box need to be to be able to fit in the street elbow and slip stub pipe that will connect it to the bulkhead? I bought a 3" (front to back) box but I don't see any way I am going to get the elbow in unless I turn around the bulkhead and basically seal in the street elbow. Right now I have maybe 1/4" of play.

If I don't use the elbow will the bulkhead by itself (with flange side in the overflow box) be able to maintain the siphon?
 
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