I made one using a IKEA bed frame and wood board from home depot. Walmart has foam pad. It looks like a real bed.
IKEA | Sofas & armchairs | Sofa beds | GRANKULLA | Futon sofa frame
a futon frame also works , if you only need for few month.
最简单的解决办法,到HOME DEPOT买木板,叫他们按尺寸裁好,当床板用。