Willow

I tried growing a willow den once. Planted the whips in a circle and wove the branches together, aiming to make a "roof". Needless to say, it didn't work! Some of the whips didn't take, while others went bonkers and reached for the sky before I had chance to pull them down into shape. I ended up with a few straggly willows in completely the wrong place.
As for kindling, I use bamboo. Grows really fast. I cut the canes for bean sticks and dry some for kindling

They had willow kits to order for making dens- they had one at the boys primary school. OH would go mad trying to mow around that