I did mine about 3 inches deep. One of the good reasons for a deep cup is that you can put additional filter media underneath. GFO, Carbon, etc.I designed a Polyfil cup for my sump and sent it over to the library. I asked for 2 copies and they quoted me $12. I asked if that was for 2 and they replied back "Oops. No, that's each." $25 to replace my 7" socks isn't bad but I realized I made it 7" deep... Not sure I need that much Polyfil. Gonna redesign a little shallower.
Also, nothing says you have to fill it to the top. Just make sure your drain holes only are on the bottom 3 inches of the cup.
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