If I were going to tackle it I would use the green florist foam blocks and a hot glue gun to form the shape of the box to fit the space, considering the positions of the evaporator and heater core; then saw it in half so you can mold seperate halves with flanges to bolt together. Set the halves on a slick surface, wax for release and glass the foam. Clean out the foam, and you have the hollow box and can glass in brackets, angles, etc to hold pieces/parts. Lots of work, but it should work OK if you know enough to position things right.