We bought a cruiser sissy bar bag at cycle gear which fits perfectly on our cooler rack. It is water resistant and has a rain cover which isn’t great but works. Secure it with a motorcycle bungee net with 6 hooks
We pack everything inside in large ziplock bags (the really big ones) and can fit 2 pair of jeans or 4-5 T-shirt’s in each one. If the bag leaks everything inside is still dry.
The bag is a Sedici Distinza T2.
We also found other similar bags made by tourmaster and Nelson rigg. The tourmaster was great but too tall and I didn’t have time to relocate the license plate before our last trip to the Smokys.
The waterproof bags from Walmart or Cabela’s are waterproof, but are not really sturdy enough to take the abuse a bag sitting on a cooler rack is likely to face. With a trike you have the benefit of having the rear wheels off to the sides and not directly in front of the bag.
The waterproof bags at places like Cabela’s are great on the rear seat, but I personally wouldn’t put it on the cooler rack.
I’m sure others may have different opinions or experiences.
As added protection, pack your stuff in SpaceBags too. They will compress and stay watertight. There is a small USB chargeable blower/vacuum called MaxPump by Flextailgear. There are two versions - one a mini for about $15 and the original for about $30. Both fit in the palm of your and and have sufficient power to compress a SpaceBag (not as well as your Kirby vacuum at home, but sufficiently).
I use water proof duffle bags from sportsman guide. Sturdy, large and not too expensive. They do work. Another option is a smaller hard case suitcase and then protect with a bbq cover or tarp. That too has worked for me when using the cooler rack
A dry sack is ideal. I have one just like the fellow who has the yellow Nelson Riggs one for sale. It is designed for kayaking I believe, hence guaranteed waterproof and not too big so it can fit on the rack. In my case I put it on the passenger seat and put my bike cover on my trunk rack as I don't want to put too much weight on the rack/trunk lid and stress out the mounting points.
As a footnote, I put clear plastic tubing inside the weather seal around the top lid of the trunk (the black weather seal is hollow). This provides added strength and helps prevent and stop the top lid grinding down on the trunk base and leading to stress in the plastic and wearing away the paint, etc. To insert it easily I sprayed it with silicone lube into the existing weather seal. Then I removed the sharp edge at the end of the tube and made it tapered so that it would be able to easily push it all the way around the weather seal at the corners where it could get stuck. Take your time and massage the weather seal and it will go. Now your seal is much stronger for the trunk lid to rest on!