There are companies that make special plastic tarps for covering hay. Bet they can tell you how to best use them too. Here is one company and you can find others on www.google.com.
When we have to stack in the field, we basically try to taper the stack. Every two or layers come in half a bale to make the taper. Bottom row on edge to keep strings off ground. Turn some layer perpindicular to the layer below to interlock the stacks. This will give kind of and interlocking pattern like stacking bricks in a wall. The overlap is the key. I hope this makes sense.