If you live in a city like London, Paris or New York, you're going to be dipper savy.
Tote bags are great for filling with all sorts but sometimes you need a city bag. 
So the answer is a bag with a zip at the top.  Its okay to have pockets inside with zips, but all that stuff in the main compartment is there for the taking, you might hug your bag and keep it close,  you forget, get distracted and then it's too late.   Some would say 'nothing is dipper proof', but we can make it awkward!
For the first in this collection, I've choose this vintage hand sewn tapestry and some fabulous striped ticking.  It will be lined in a metis (linen and cotton mix) and have leather handles.  I'll decide on zip colour and pockets as I go.   Size of the finished bag will depend on the size of the tapestry.
 
anti dippers fabrics