I dug up a clump that had been earthbound for at least 4 years last Fall. It was 15 inches wide, a foot deep and had some of the hugest tubers I've ever seen- I bet two pounders. Could serve a family of 4 with ample butter and salt!

I would try for that size of hardware cloth lining and see how it works after 3 years or so.
Good luck with the gophers. Moles were an earlier problem for me, but they seem to have moved on, thankfully.

Do gophers do this to your lawn and gardens? This was early March and there were plenty of mounds inside the garden beds as well.

One year I stood in amazement and watched a 2 ft tall dahlia jig left, then right, back left and right. Thought I was losing the last remnants of my mind! Seconds later, out of the shredded leaf mulch popped a mole!
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Thanks poochella for those measurements, that is exactly the information I was needing. I don't have a lawn but yes they do that to my garden. They are devastating. Last year they ate three 50' long rows of perennials in a matter of a few weeks. I built a homemade gopher blaster this year and I expect to be rid of them by the end of summer. Blasting their tunnels and caving them in to rid my garden of the network they have mined throughout the years should help get a handle on the situation. Combined with trapping and wire cages, I may be able to keep a perennial around for more than half a season. Someday I will tell you about the corn stalks they were pulling down in their as I stood there and watched. Dana