Maybe the Arriba(?) bag for a light ball(?) that was recommended for the Genz-Benz Shuttle 3.0-10T here on Talkbass might work for the Doubler/Wizzy10 as well. The sizes might not be that much different.
I use a cajon bag from Thomann (their own label, the cheapest they have, I ordered two and that was even cheaper than one Arriba) and it works well for my Genz-Benz Shuttle 3.0-10T but the cab should not be wider or deeper than the Genz-Benz cab to fit in there. The drawback is that if you use the handle and are a bit more on the shorter side, the bag might get in contact with the ground. But the shoulder strap is great and well sewed and there is still some room on top for a effects unit, preamp etc. A bit of foam on top of the amp and nothing gets damaged. Also the big front pocket can take sheet music, cables and mobile devices. Sorry, no backpack straps, but you can shove the bag to your back with the shoulder strap. I ordered several bags from Thomann (could easily send the stuff back I won't like here in Germany) checked them for my Shuttle combo and at the end it was the Arriba (with a different name here in Germany) and the Thomann cajon bag. Not every cajon bag could be used, I checked the sizes before and even the Thomann bag was a bit risky, but in the end that was the best for me (with the tight budget).
The padding on the cajon bag was a bit better than on the Arriba and the cab was easier to insert too, but since it is longer/higher, there is the potential problem with ground contact.
