Really need recommendations of what dimensions (length, width, and depth) should be.

Which is cheaper, from catalogs or building myself? Somebody suggested using lumber from pallets, but this isn't an option, as material from pallets aren't available.

What sort of lumber? Need sturdy and long lasting, but economical too.

Lumber available locally, however there is extra charge for cutting to size.

Labor is free (building myself).

Would be for melons and squash; maybe tomatoes, peppers, and eggplant.