Cushioning Options for My New bondage Platform

My bondage platform project is progressing nicely, though I’ve encountered a minor issue. The pine boards I purchased are quite hard on my knees for extended periods. Consequently, I need some sort of cushioning. I’ve read that others use thick rubber mats (originally designed for stables), but I’m unsure if these provide enough softness or fit within my budget. So far, I’ve spent roughly 125$ (including the cost of ten new padlocks), so it would be great to find an option in the 30-45$ range. This thread details what I am constructing: the referenced community page