This is the gantry with the supports added, it makes it a little easier to understand what I'm suggesting (the view is from the rear side):

The red box section will be mounted to some SBR rails, the orange box section forms the main parts of the gantry, the yellow bits are struts made from steel angle and the grey bits are 10mm steel bar.

Click image for larger version. 

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Edit to add, click the piccy for a larger view.