Many people have a problem with seiza. Most of the time they have stiff ankles. If ankles are stiff it is a torture to sit in seiza not even a second! This might help. Might not. I do not know. But there are some things that might help you to sit on seiza.
If you are not used to this, it will hurt, especially on the top of the feet. This is a pain you are feeling on the skin of the feet. But if your ankles or knees start hurting, the inside body not the skins, that is not good. Please stop. You have to “train” to sit in seiza.
Hope this helps!