If you want to be able to secure it and keep the contents hidden, why not consider a type of drawer vault similar to the types made by companies like "Truck Vault" or "Goose Gear?" You can of course source something from one of them or another manufacturer, or take the time to make your own. The Goose Gear products use a product commonly referred to as "80/20 Quick Frame" which is very light, endlessly customizable, and very affordable. If you've ever built anything from Ikea then you can easily make your own 80/20 storage drawer for your Subaru. Once completed you can take it a step further and place like items into small labeled bags that are easy to remove when needed but help keep everything neat and tidy. I'd think that something with two drawers, six to eight inches tall and the width of the rear of the wagon that is deep enough to just end at the rear seats would provide you with a considerable amount of space and organization. May be "taking it too far," but I'm the type that likes to carry allot in my vehicle but want everything safely in it's place so it doesn't become a missile in cases of emergency maneuvering and collisions. Here is an image of a single drawer unit that someone made out of quick frame that is probably ten to twelve inches tall.