wet/dry / sump issue?

[da1jewfish]

I just got a 125 gallon tank and will probably set it up for FO predator tank.
The problem is that i planned to use a 29 gallon fuge for the main filtration, but it doesn't fit under the stand. So I was thinking about cutting the back opening bigger, but I rather not.

what size sump should I use in the stand?

thanks all!