It's been a while so here is an update on the Stug III F. It's finished! as i had been building and painting it before i started this blog, i have put together a series of pictures about the base. 

 From a sheet of plastic card, i measured up suitable dimensions and cut out. Make up a square with right angles and insert corner strengthners. This creates a simple but strong box. Insert a rough cut bit of polystyrene and trim to shape. i decided to try something different with the base and so i used a textured aerosol paint, which i must say looks great. I certainly be using this technique again.