Currently, all custom containers can be accessed by just having the right block. It could be more interesting to add container key custom items: Players can right-click with it on the right block to place a custom container inside that block. Doing this will consume the container key, and that key will be dropped once the block is broken.
Custom containers can only be accessed on blocks that have been activated with the right container key.

I think it would be better to attach custom containers to custom blocks. This issue is basically a work-around designed when this plug-in didn't have custom blocks yet.