As you may have realised, the working area on the Hopwood fiddle yard is minimal; the only way was up. Early on in the build I'd factored in a shelf. This would double the off-track storage space - under would take longer CCTs etc. parked end-on. On top would take standard length coaching stock or locos.
A simple enough addition: lengths of strip wood stuck to the uprights and an MDF shelf approximately 111/4" x 5" dropped on top. Not that there will be an enormous amount of stock used, but this does give a little more flat area if need be.