Good progress so far Bob. For the undercart doors, get some plastic sheet from a model or model railway shop, available in various thicknesses. Draw the shape of the doors onto the sheet, and cut out by scribing, continuously, around the pencil lines, until the part is free. To create the curve, you should be able to bend the part around a finger or, if having problems, heat it in hot (not boiling) water first.
If you can't get plastic card, then 'paper' card, such as found on greetings cards, might be easier, and can be laminated to increase the thickness if needed.