Just a thought: Building the sub box into the panel will make it much more difficult to remove that panel when you need access behind it. We built a stand alone sub box and put it behind the panel and then covered that hole with a general sub grille and lights. It will also be a more difficult to seal that panel to the box you build.
Those hosed and cables should lay down flat and run to the outside wall, at least they did in mine. You can add small standoff legs to your box which will allow the hoses and cables to run underneath.