Cut the frame from plasticard sheet rather than trying to make it from strip, build up the thicknes from thin sheet it's easier to cut.
This is how I build up window frames for my lifeboats.
1: Bottom frame marked out on plasticard.
2: Top frame marked out on plasticard.
3: Bottom frame cut out/sanded to size.
4: Top frame, centre removed and glued onto clear styrene glazing ( CD case!).
5: Top frame/glazing cut to final outside shape/size.
6: Top frame glued onto bottom frame, painted and ready to fit.
Mike