3d print a divider for your case?
Here is someone who still sells the SOLARFORCE box. It says it fits 6*18650.
But it also says that 4*26650 fit in, so maybe some 3D wiggling is possible.
Bmengineer. I've been thinking about this as an alternative. Cut them in half and glue them. But then again it wouldn't look good.
The solarforce case looks like an open case without the dividers, unfortunately.