A good custom-made cover takes all the hassle out of Ribbing in my experience. No messing around with ropes trying to tie your PVC tarp down, keeps everything clean, dry and safe (out of sight!!)
Got ours from
www.starspring.biz which was quite convenient as they only need one or two measurements and a photo, not the boat itself. However,
most upholsterers, sailmakers will make you a decent one. Be sure it's breathable though unless you want condensation in your instruments and gauges.