I thought his thread was about strapping down a dry bag on a rear rack. The Ortlieb Rack Pack Dry Bag and pannier examples are designed to connect to each other and cinch down for security. No additional straps or bungees needed unless you are talking about holding down other loose things back there.
I long ago eliminated unreliable (in my experience) bungee cords in favor of nylon straps with Fastex or similar buckles. As with BLYTZPK I transferred my experience with hold down straps from sea kayaks to bike touring with great success. ( I do recognize that improved design and materials have made for better bungee cords and hooks but this old dog is probably not going to experiment with them.)

If dry bag tie down is still a topic of this thread I would offer a different solution. Many newer lightweight dry bags are somewhat fragile when exposed to UV sunlight and abrasion in contact with racks and other fittings. They lose their dry bag status quickly at times. To protect the dry bag I enclose it in a stronger larger stuff sack on which I have sewn on several web straps with buckles. Rather than attaching it perpendicular to the rack I align it with the axis of the rack and thread the straps under the rack rails and clip the buckle ends together and cinch down tightly. This setup has worked for me over the last thirty years on and off pavement. And my dry bags have lasted for decades as well.

Over the years I have reduced the weight and volume I carry to the point that two smaller front panniers and a medium sized dry bag on top of the rear rack handles all of my touring needs for short and extended tours like the Divide Ride. The front of the stuff sack shell holds my sometimes wet shelter and rain gear while the dry bag carries all clothes, sleeping gear, pads safe from moisture.

Just my 4 cents worth.