it will be built in one day.
So far do this:
CREATESCREEN 0,0,480,320
USESCREEN 0
WHILE TRUE
.. draw as if it was 480x320
USESCREEN -1
ROTOSPRITE 0, (320-480)/2, (480-320)/2, 90 // or 270 for other orientation
SHOWSCREEN
WEND
You have to rotate the mouse coordinates, too.