I have two Radrovers with Saris Superclamp 4 platfrom rack. I removed the outer platform arm when I travel out town with my Rads to Grand Canyon a few years ago. I purchased the Formosa bike cover from Amazon for about $45+$18 for shipping:
https://www.amazon.com/gp/product/B000WENDUS/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
Not sure how much longer the Radwagons are compared to the Radrovers with the 4" fat tires? I had plenty of room with the cover to completely encase both rads and platform rack to the hitch connection. Lots of straps to tighten down the cover and translucent panels near the tires for rear vehicle light to shine through. This cover also folds down in fits in a case about the same size as a small pillow on a kid's bed.
I didn't run into any bad weather on the hwy and didn't have to drive with the cover on. I used the cover at hotels for extra security and to keep the morning frost off the ebikes. I did this for some of the hotels we stayed and I couldn't take the ebikes inside for the night.
You could always try a few covers with the radwagons on the hitch to see what fits and send back the one that don't. I really like Amazon's 30 day return policy for things like this.