Our problem is holding down the covers. Cardboard is great inside the polytunnel, but outdoors it needs manure or similar on top, then something to keep out the light. Bin bags we find too small, bale wrap too flimsy, so originally we used an old tarp or two. Then, when we started using alternate patches year and year about, we invested in a large, heavy tarpaulin which fits the whole area. It still has to be held down with breeze blocks, old stobs or dead pallets, and the terriers still do their best to rip it to shreds, but it's lasted several years already. One drawback of using an impervious cover is that it collects water, which is heavy on the soil below and compacts it, once the manure part has rotted down.