I've been down all three paths over the past 25 years, uncovered, custom cover (we went the tyvek route),and tarp. The worst set up was the custom made cover - was always sopping wet even underneath it. Now we use high quality tarps that are white on both our fiver and our TC. We keep them up off the EPDM so ventilation occurs between the tarp and the roof membrane and we allow air to pass up around the edges. The tarps have been lasting about 2-3 years longer than the custom covers.
This has turned out to be the best set up for us and has saved boatloads of money in cover cost (not customized to fit) and the rigs and roofs seem to hold up quite well. The multiple grommets allow for a good secure tie down. Different folks will have different results and benefits from other set ups.
When I first started this process of using tarps about 25 years ago, I used bungees but was getting scratch marks on the side-- so we stopped that immediately and went to parachute cord and wrapped the grommets with duck tape to prevent any additional scratching from rubbing.
Been going down this road for quite a while and it has worked best for us