We use the stock FWC mattress on top of our Froli travel system with a thin mattress topper. Top still closes (even with light bedding) and the froli made the stock mattress much more comfortable. Even though I like a firm mattress, the FWC mattress was too firm, leading to a sore hip after multiple nights.
Edit to add: This is on a flatbed, so the top closing part may not apply to a slide in.
Edit to add: This is on a flatbed, so the top closing part may not apply to a slide in.