So before I get spoilsports flaming me and telling me that 'this isn't a real instructable because you haven't built anything' etc., let me start by saying "I know".
I am a huge fan of Instructables and have the greatest respect for the multitudes of creative, clever people who are making some of the most amazing products both simple and some massively complex. This is an idea that I accidentally stumbled upon whilst in the process of sourcing ingredients for another 'way more complex' instructable that I hope to undertake in the near future. I just felt that it was the kind of idea that others may find useful.
I bought a set of bookshelf speakers from Ebay for the incredibly bargain basement price of $3.25. I kind of felt guilty taking them from the nice man and only having to hand him 2 gold coins. They are really quite good quality speakers in a very well made, solid and weighty box.
I have been frequenting Ebay to source inexpensive speakers to be used in one of those hacks/mods where one builds a mobile, 12v battery powered, self contained sound system into an old retro suitcase or similar. Let me tell you that I have found some absolute bargains and now have a sizeable collection of speakers, some of which are the 'real deal' and deserve far more than to be put into a 'gimmicky' suitcase mod.
Anyhow, I digress. After unscrewing and removing the speakers, I was left with an empty box that with the front grille re-attached, looked for all intents and purposes like ... a bookshelf speaker. The front speaker cover simply pops on and off making it relatively easy to fill the vacant space with giant wads of cash or valuables or USB keys containing highly sensitive information stolen from a government agency of your choice.
I can't speak from experience as it has been a long time since my 'breaking and entering' days, but I'm pretty sure your average burglar is going to dismiss the speaker as being a relatively cheapish looking piece of furniture that would not be worth the effort to abscond with. One of the attached photos shows another, much larger speaker box that I have ransacked for parts. I can see no reason it could not also be used for the same purpose. (please note the beer bottle in the photo is only present to demonstrate scale ... oh and also it was a 38 degree day [that's 100 degrees Fahrenheit])
I have decided that the speaker box looks (and sounds) good enough to not destroy the second one. Instead, I think I shall make another Instructable and turn it into a portable, self-contained music box. I shall pop a handle on top (it really is deceptively heavy for its size) and build into the box an amplifier, rechargeable 12V battery source (LiPo), some led lighting (or maybe electro-luminescent wire/tape), some solar panels on top, built in FM radio/usb/sd card MP3 player, with audio line in for iPods/phones etc. and maybe even bluetooth.
Now THAT's a proper Instructable.