I prefer bland fruit if I want fiber, as most of the fiber there is soluble, whereas most of the fiber in leafy greens and tubers is insoluble. I don't think we're meant to be consuming large amounts of cellulose, that is for hervibores.
Have you tried this? Tomatoes, cucumbers, bell peppers, etc.
There are some starchy fruits as well, such as squash/pumpkin, which you mentioned, very unripe green bananas or plantains... Actually almost all fruits are starchy when unripe, it's just a matter of how palatable you can make them...
Usually if I want raw starch, I go for corn on the cob, carrot juice, or beet juice. Celery juice is great too but it contains very little starch, I have it more for it's refreshing and cleansing (detox) properties.