Once you have located an appropriate suitcase do anything you need to prep it for modification.
In my suitcase there was a divider that wasn't going to work with my plans for the finished case so it had to go! Some pulling (with the pliers) along with a little bit of cutting and it was free. I tore the fabric covering the hinges in the process but I had already planned to cover it so it didn't matter.
The divider clipped into the frame with two rotating clips. Those I could not remove and had to work around.
I glued fabric into the case in places that will show at the end of the project. Gluing new fabric can happen at any point in the construction process right up until you place the wooden boxes into the suitcase.