This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
Rope nets or iron bars or even a grate.
This is a pretty versitile filter with controlable bevel and shadow.
net bars rope iron mesh grate grid string fabric material overlay tent
This filter has earned its author one of our rewards. They are given to the authors of popular filters and include a free copy of Filter Forge.