sketchup is pretty basic, its not designed for engineering drawings. If its a one off job, then get some paper and a ruler, and draw out a good set of drawings for each face, and a sketch explaining how they go together. Hand this to your manufacturer, then pay them for a couple of hrs work and get them to model it in cad. Any half decent cad package costs lots of money, and takes time to learn.
As an engineer I would always prefer a good concept sketch to badly modelled cad files from a novice. Also if your manufacturer is worth their salt they will probably suggest improvements as they are doing this, where as if you send finished cad files the temptation is to make the parts "as-is", even if they can be made cheaper with a small tweak.
If you want to sell this, its also probably worth getting at least a few hours consultancy from a mechanical engineer, to go through the whole design (not just the chassis), and advise you how to make it in a cost effective way.
EDIT: one other option that might suit you better, is if you can sketch your sheet metal faces in your electronics package, and export a dxf that is easy enough to import into 3d cad