How about a very porous Hypertufa bottom layer!?

I think the chemicals for the galvanizing will leach quit rapidly. Every time I work with galvanized materials I get this gray residue all over my hands and clothing. I don't know what chemicals or elements these residues are but most likely more toxic than the lime leaching from a cement based material.

It's very slow. Galvanized nails are used in exterior applications for a reason, after all. You'll notice that such nails rust away mostly at the heads where the galvanization was compromised by the hammering. Also woods like cedar and redwood have compounds in them that do eat away at the galvanizing, so nowadays we use stainless steel fasteners for those.

I use galvanized wire in all 20 of my beds. I'd never be able to garden without it because of the gophers and moles. For several years I tried growing without wire, and it was a waste of my time. The trick is to line a hole at least 18 inches deep because your roots need room to stretch. Gophers and moles will dig all around the edges of the wire, and sometimes under it, so be sure you don't leave any open edges where they can squeeze in. Good luck. Lyn

Thank you Paulyn, good to know.

Do you use 1/2 inch mesh or 1/4 ?

The smaller the better. I've used 1/4" with success to keep gophers out. That said, I'm reconsidering the use of galvanized since I'm concerned about leeching of chemicals. I'm looking for alternatives and considering a thin plywood.