As long as there have been gardens there have been pests, but the possum is about as tricky as they get.  Not only do their oposable thumbs make them very capable thieves, but they are also easily adaptable to deterrents set in their path.  Take a look down the pest control aisle in the hardware store and you will find a suite of products ranging from chilli sprays to high-frequency audio devices- all claiming to be the definitive solution.  Unfortunately, possums seem to have now found a fondness for chilli/garlic sprays and these audio devices are much more likely to annoy your poodle than save your lettuce. 

As is often the case, the best solution is also the simplest.  The only time-tested method to protect a patch is by preventing access to it.  Possums being obsessive and habitual creatures, prefer to take the same route into the garden every time.  The first line of defense is to disrupt their entry points with barriers.  The use of either a physical or "stink bomb" barriers work best, but may be a lost cause.  If they get past your fortifications, the only true protection is a garden net.  In the case of a raised garden crate, simply affix a total of eight brackets to opposite sides of crate (that means four on each side).  Create a sturdy/light frame by bending two 3m lengths of electrical conduit from one side to the other and slide them into the brackets.  Finally, throw a net over the top and secure it at each side using standard screw hooks from a hardware store.  Eureka!  Until the possums figure this one out, you will be enjoying backyard gardening bliss all year around (this will also protect your patch from birds).  

Music by Los Amparito