Thanks Julian, with your hints I cracked it!
fmWindow = New Window( "Fit Model Dialog Example",
H List Box(
pb = Panel Box( "Launch Window",
fm = Fit Model(
Personality( Standard Least Squares ),
Emphasis( Minimal Report ),
SendToReport(
Dispatch( {}, "", PanelBox, {Visibility( "Collapse" )} ),
Dispatch( {}, "", Panel Box( 2 ), {Visibility( "Collapse" )} ),
Dispatch( {}, "", Lineup Box( 2 ), {Visibility( "Collapse" )} ),
Dispatch( {}, "", List Box( 23 ), {Visibility( "Collapse" )} )
)
)
),
Text Box( "Other content here" )
)
);
By just collapsing all boxes I don't need I could eliminate the parts of the window I do not want!