BUMMER! it doesnt sound like you got the right model....
i recommended the CPR bakpak SR3 model, which was designed to go in the back of a 29g biocube.
http://premiumaquatics.com/aquatic-supplies/CPR-SR3.html
" Easily slides inside of the Oceanic BioCube 29 sump area; fits in the middle chamber of the sump section of the BioCube 29 without any modifications. The lid closes as originally manufactured and the collection cup can be easily removed for cleaning through the rear access hatch."
hopefully they let you return the regular bakpak...