Potty learning is an exciting step in a child's life and we're happy to help with some suggestions.
While there are some products out there that are marketed specifically to protect a car seat cushion while a child is potty learning, we recommend against the use of these as most go up the seat back and obstruct the harness straps. It's otherwise fine to use something thin and absorbent underneath your child in their Foonf provided that it does NOT interfere with the crotch strap or harness straps.
Many parents have had success using either folded microfiber towels, as microfiber is very thin but absorbent. If you or a friend used cloth diapers, their microfiber inserts tend to be the perfect size to sit between the back of the seat and the crotch strap.
Whatever you choose, it's most important that it remains flat on the seat cushion, does NOT curl up the back, and does NOT interfere with the harness.