Of course, the "NEXT" model can also use "solid" tanks. The picture is just a preview of what Damiano used, but it was not said that such a tank should be used.
Last weekend, Damiano made another ten starts and as you can see in the picture, the tank was replaced.
The reason is that such a tank has a larger volume, so the flights lasted over ten minutes.
Fuselage of the "NEXT" model is huge, so any tank can be fitted, there is no need for special custom made shape.
Mirce