Because willow grows so quickly - the Salix Viminalis ‘Bowles Hybrid’ short cuttings can provide 3 or 4 whips of 8 or 9 (or more) feet long in one season - you might like to ‘grow your own’ structure ! You could either plant the cuttings in one place and simply cut the whips and plant them where they are wanted in winter - and the cuttings will give you more whips the next year . . . and the next . . . . !
OR you could plant the cuttings where they are wanted, for example, as the perimeter of a circle for a dome/playhouse, or in two rows to make a tunnel. It will take a year longer to create a structure than planting with long whips - but it will cost less and you might prefer the idea of a less formally planted structure.
We had not actually created this type of structure ourselves before - because we have plenty of long whips whenever we want to make something - but we thought that we would try some in our ‘experimental patch’ as several people ordering cuttings said that they were planning this sort of project.
For a small child’s den we planted 18 x 10 inch cuttings of Salix Viminalis ‘Bowles Hybrid’ in a circle of about 5 feet in diameter at about 9 inch spacing, leaving a gap of about 3 feet for an opening. We planted in January, and the photograph below shows growth in early May.