Drew, it sounds like you can take any of several approaches.
First, you can make another sheet-metal body within your folded walls and use the convert entities sketch tool to define a floor shape that match the inside surface of your wall.
You can also do this in the context of an assembly.
Lastly, if you understand the variability of the floor you can use equations at the assembly level to drive both the wall and floor geometry.
One question. Does the example you posted show the sheetmetal walls completely folded or is there another step to the process?