Playpen sleeping baby advice help

When I put my daughter's crib sheet on her playpen mat (what she sleeps in) the mat kinda folds up at one end no matter how I put the sheet on because the mat has separate boards inside to fold it for travel causing the force of the sheet to fold it at the foot of her playpen I've had people tell me to put a crib mattress inside it over the thin board mattress but I can't afford one can someone please give me ideas on how to fix this or what you guys do who also use a playpen as a crib