Used Crib mattress (Upper Darby)

Hello free family, i am making a window day bed for my two fur babies and im looking for a crib mattress that measures 55in x 10in x 5in.

Preferably memory foam as I intend on cutting it to fit the window ledge.
I have to build a box to contain it as it will sit over my radiator box

I currently have (2)5 below pillows but they dont understand the assignment lol so the still sit on the cold window ledge to look out

Thank you in advance.