Hi idoque_le,
I don't think the shape u r talking about can be manufactured by sheetmetal bending process. It can be done by Punch and die (forming) process.
Therefore u dont have to flatten it. Anyway if u really want to do it, use unbend option instead of flat pattern. select the bottom surf when prompts for fixed geometry and select the four round surfaces which connects the sidewalls when prompts for surfaces to deform. but u won't get accurate results by this above method. (u get a polygon shaped flatwall).
If u reallly want to create a component like the one u asked for and flat pattern it, u should start from sheetmetal directly instead of solid modelling.
If any other information needed please reply. Thanks.