skimmer size

I have a wet dry filter with 3 chambers and the first chamber I assume will house the skimmer. I wanted to use the coralife 125 SS but there isnt enough room for it under the cabinet so I am going to start looking for a smaller skimmer that will do a good job....The space in the first chamber where the skimmer will be housed is 8 1/2 by 5 inches I.D. what would be an adequate skimmer for this chamber the tank size is 55 gallon. Thanks for your time....
Danny

 

You could use an external skimmer and use the 8.5x5 space as a fuge. Or maybe some of the following would work in the space:

AquaC Remora Hang-On Protein Skimmer

CPR Bak-Pak Protein Skimmer (that's what I use on my 55)

Detailed Product View

Or, I have always wanted to try this DIY external skimmer-

DIY Skimmer

 

I like the looks of the Aqua C Remora how much over head room is required to take the cup out to empty. I have 9 inches even above the fuge to work with....It states the heighth but not how much actual room is needed to remove and replace the collection cup....
Danny

 

I think they need about 6" clearance to remove the cup.