These bags (see photos below, courtesy of Co-Motion) are the best stoker bags we've seen, and it seems like we've tried them all. They are easy to mount, with 3 Velcro straps, 2 above the bag and 1 below, to secure it to the stoker's top tube. Together with the large size of the bag, it is the 3rd strap that really makes this such a practical bag - it doesn't move around when the stoker gets into it while underway. One-handed use is a breeze. As mentioned, it's large enough to be practical. My stoker stores a Canon Powershot G1X Mark II in it.
Retail price for the bag is $49.00. Sales tax is additional for Washington residents. Shipping is extra. They are light, so shipping cost is reasonable.
Let me know if you would like to purchase one and we can arrange payment and shipping. Thank you!!