Some place on this site, there is a long thread on "hidden spaces" on our boats, but my Google-Fu seems to be weak today.
Although I don't have an E27, here's some random stuff:
1. Less is More. Always try to keep the cabin organized and un-cluttered. Well... some day
Even more-so for the deck. As much as possible, weighty items should be stored below the waterline, for stability.
2. Many people have explored the sealed-off spaces between the liner and the hull. E.g.
http://www.ericsonyachts.org/infoex...aliquots-of-progress-Stbd-side-stowage-Part-1
A couple of points about opening the liner: In these 70's designs, the liner is supposed to provide stiffness to the boat and reinforcement to the hull, so be careful not to weaken it too much when cutting holes in it. Rounded holes are probably better than square ones, which might lead to cracks propagating. That said, the liner blocks access to lots of deck hardware that was installed before the hull and deck were joined, so at some point, holes must be cut anyway, to maintain that stuff. I like screw-in deck plates for some of that. An argument can be made that there should be some way to access
all the spaces behind the liner, for potential damage control in an emergency.
3. There is usually a large empty space behind the galley, especially if a range is installed. It's not easy to figure out how to use that space. I'm considering a hatch in the bottom of the main cabinet, but one would have to unload the cabinet to open it. Or pull the range out. So, certainly not for stuff you'd want to get at frequently or quickly. Maybe electronics black boxes?
4. How much "stuff" are we talking about? Live-aboard level stuff? A lot of people don't need all of the bunks on the boat, so one or more of them gets used for storage. If that's the case, one might as well install decent cargo tie-downs (which can be hidden behind cushions) and/or secure shelving, as opposed to just a pile of stuff on a bunk. A recent abandonment of a nice boat off the coast of WA was prompted in part by scuba tanks coming loose from the quarterberth and books falling off shelves (and clogging bilge pumps) after a big wave. And those people were experienced cruisers. Probably a good idea to consider what would happen to any of your stored stuff if the boat goes sideways. Or upside down?