You could add sides if you wanted to but you don't have too. I have a new double hitch carrier and a single hitch carrier. In both cases, the rear tires sit more on top of the rail rather settling down into it. The new double did not come with straps to secure the tire to the rail but suggested it. In the picture of my single, you can see I used a strap. If you strap both ends of the TW like I did you may not have to use straps but could if it brings you peace of mind.
I haven't nor will I use straps on the front tires of either carrier.
We were using the carrier as a work stand but I wanted you to see the strap. I quit using the strap for shorter trips but for long trips, I still used tire hold down straps...for peace of mind!
I haven't bought the rear straps yet as I just put this together a week or two ago and am just practice loading. I'll have rear tire only straps for trips.