I use the same grey / brown 10x20 tarp as Andy. Most of the time mine is on the trailer, without the frame (I paddle more than I row). I used to bungee it down, then last year ran a piece of nylon rope in and out of the grommets with a loop on one end. I tie the loose end through the loop to secure it. To take it off, I have to pull the rope out about 8 grommets. Total cost: $35
Never had an issue with wet gear, but my trailer is elevated off the bottom of the trailer and just the tubes sit on 2x12 boards.
Claimer: Someone that makes a claim that they have been there and done that, can do anything you can do better than you. I hate "claimers"