I too have the Wally World special. My wife and brothere in law also. I strap mine onto my back rest of my Corbin Solo Touring seat. Haven't had it budge yet. My back rest isn't very tall either.

 

I have a Harley backpack, not specifically designed for the sissy bar. I have the Harley sissy bar on my CVO Limited. I set the pack on the passenger seat, then crisscross the shoulder straps behind the bar, and wrap them twice around each upright. Then I secure them with the chest strap, clipped together on the front side of the bar between the bar and the bag. That holds the backpack securely in place for travel. You could use any backpack with a clip together chest strap the same way.

 

I use a $5 bungee net (typically 15" X 15" when not stretched) with the backpack sitting on the rear seat, strapped around my sissy bar every day going to work. Never had a problem. When I lived in Austin, I also used a similar, cheap bungee net to load 2 full brown paper sacks of groceries strapped on the back seat in front of the sissy bar, and only once or twice in 2 years of daily riding did I have anything fall out. Cheap and easy fix.

 

I`ve got a Harley backpack also. I bought it 5 years ago when I read this thread, lol

 

I have a very large “sea bag” I got at a surplus store almost 10 years ago. It’s waterproof, durable and like I said before It’s massive. I think I paid $15 for it then. 2 bungee cords will hold it to your sissy bar no problem. I rolled that why for a long time before I bought bags.

Also, if you put enough stuff in it it makes one hell of a backrest for long trips.