1/2-inch hardware cloth is the best - but it is very stiff and hard to work with, you can't mold it to fit irregular shaped beds. If your beds are simple rectangles, it works fine. Mine lasted at least 12 years, until I removed the raised beds it was protecting. Make sure it comes snugly up the sides of the boards (or whatever you're using), or they will squeeze through.

Chicken wire also works, but it will rust out more quickly - maybe 3-4 years. I use chicken wire to make planting baskets for most trees & shrubs; by the time it rusts out, the plant should be well-enough established to survive the occasional nibble.

You can get either material at any hardware store or home improvement center (HD).

I found some small-hole plastic fencing that I'm going to use for just such a bed. I'm hoping it won't disintegrate like the metal wire does.

The only problem I foresee with using plastic is that gophers have very sharp teeth - and if they want what's inside badly enough, plastic won't keep them out!

I put down weed cloth and then a very thick tiny guage plastic used for this. I bought it at Lowes in Folsom. Yes the chicken wire does rust out and rot and the hardware cloth looked next to impossible for me to handle. I have 3 raised beds all in different areas and tons of gophers w/o a problem so far - praise God! Investing in a king snake is a great idea too.

The hardware cloth is fairly easy to work with, providing the boxes and cloth are similar in width.

I rolled out the cloth, cut it to size with a simple wire cutter, then stapled or nailed it to the bottom of the box, flipping the box over the area to be planted. If the sides of the enclosed area is heavy timber, etc., then the hardware cloth needs to be laid flat on the planting ground prior to placing timbers, and edges turned up somewhat for securing/stapling, etc.

When making my initial planting areas, before the boxes, I worked the soil that would be underneath, putting in a lot of material that would compost down eventually. It provided much better drainage and the added soil/compost worked down through the hardware cloth providing deeper loose soil that created better drainage above my hardpan soil.

The hardware cloth is going on its 5th season, with no signs of varmints getting through. When the boxes begin to show signs of weathering from soil, I will turn them over, nailing the hardware cloth on the reverse side, providing there are no worn areas.

Just my 2 c's.

Bejay