Hi Ben, some answers for you
Firstly about design, where you could make your life easier. Maximising strength without adding weight is best achieved using contours and returns rather than flat featureless panels. Next best method (based on strength/weight) is to bond (or glass in) a hollow rib section, like an automotive hood. Next best is to bond/glass in a foam core, and note that the benefit is derived from the section depth of the fibreglass not the core material.
Core material is not expensive but there are many alternative free sources that you could use.
On the CSM weight question, you are right, its metre squared in Europe. I’ve just checked a US site to help you convert but its quite complicated. Think that 6oz cloth as shown on this link
http://www.fiberglasssite.com/servle...%22/Categories
looks too lightweight for use as your principle material. The same site shows 18oz woven roving material which looks closer to the size I would suggest, but there is a disconnect in the weights since this site only has two weights of CSM (1.5 and 2.0oz) – puzzling!
Regarding the finish question - A topcoat will do what you want, but I have not used personally since all my work has been done from normal moulds. Here’s a link
Gelcoats & Topcoats, White Topcoat, - CFS Fibreglass Supplies
As a final note, my recommendation is that your panels only require a skin thickness of 1.5 – 2.5mm and this would be two layers of 450g mat, which I would suggest you do in two visits rather than rush one lay-up and compromise quality. Then release from your former (its not really a mould) and see how stiff it is. Bond on ribs and mounting blocks before doing the outer surface. I have a couple of other tips for you based on your images, if you need more help
Regards
Pete