Those Donkey Boxes look to me like they're made of the same stuff the post office uses for its boxes and trays. They surely get a lot of use and abuse, but I don't know if the USPS uses them because they're durable, or just cheap.
Since they're new, we need a few field reports. If you decide to use them, please do report on how they work and how durable they are.
A few other concerns:
Since you zip tie them to the rack, it's not going to be easy to unload the bike. For instance, if you decide to stay in a motel for the night, and the only room they have is a second or third story room with no stairs, how do you get 60-100 pounds of bike plus luggage up there? (Happened to us twice...) If you decide to unload the bike, do you have to re-attach the boxes while they're full, or do you re-attach the bike, then lug everything down and load it when it's on the bike?
Likewise, we stayed a couple places where the bike was parked in a shed, or otherwise out of eyesight. What do you do then?
How rugged are the zip ties? I can see using them for a week long trip on pavement. What about 10-12 weeks, over some nasty, bumpy, roads? Do you need to replace the zip ties periodically? Will the attachment points in the box stand up to the load?
Finally, what do you do when / if you camp in bear country? Panniers easily detach and go in a bear box. I think you're left with either detach/reattach the box in the morning, or trying to unload the box, and just deal with whatever bears or raccoons do with good-smelling boxes.
If you do decide to try the box, I'd get Wayne's number at thetouringstore.com and put it on speed dial for the possibility that you need an emergency fix, and the replacement panniers positively have to be there overnight.