PetEdge I think or maybe Foster and Smith, have a puppy play pen for whelping litters. For the 48" they use two trays, 48x24". I think you could probably buy the replacement trays for that separately.
Another thought would be a rubber stall mat, from I think Tractor Supply. I think they are 6'x4', but you could probably cut one down to size, and I think they are about 35$ on sale.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC