They appear to be cylindrical cones around around the feet going to the knees. Cardboard would be fine as a base as you can layer fabric over it.
But if you want it to be slim around your feet, then I hope you like wearing high heels as that's the only thing I can think of to slim the feet area to make such a small cylindrical area.
I hope this helped.
When in doubt, just take the next small step.
With that next step take a deep breath. It calms the mind.
When the mind is calm you realize the best is still yet to come.