You might explore lasagna gardening.
However, worms don't migrate great distances so if you don't already have a good local population, they won't magically appear so a worm purchase is likely. You might still consider an in-ground bin no deeper than 18" or so (certainly not 4-5 feet!) to help moderate temperatures and moisture. Consider covering and lining the bottom/sides with hardware cloth to keep critters like moles/voles out.

i keep my worms in my garage and have not had trouble.I have a C.O.W and a homeade bin from a rubbermaid container.
Here is the best trick I have found for keeping them damp. Always have about three inches of newspaper strips on topof food scraps.They must be soaked with rain water or water that has been allowed to sit for 24 hours(to get rid of chlorine).Lift the strips to add food and add more strips as needed.If you are feeding them fresh food scraps weekly you should have MORE than enough moisture.Fluff only about once a week.
Red wigglers are best to eat your garbage.They also adapt well to bin living and reproduce prolifically..
If you do not want to bother with raising worms but want to attract garden worms the best way is to add sheets of wet newspaper underneath your mulch.In about three months lift it and you will see garden worms.
Good Luck
Here is a photo of a homemade bin at an organics talk I gave
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