I got this: https://www.harborfreight.com/foam-kneeling-pad-56572.html?e.... For $6.99 cheap at twice the price.

I just use a piece of styrofoam that was padding in a package.

I have a couple pairs of Fjallraven Vidda Pros, and they have double knees with an opening to slip a foam pad inside (which they also sell). Carhartt has a similar method with their work jeans. I’m sure this could be DIY’d as well for the sewing-machine-inclined.

price just dropped to $6.29