The "bed" us just a mattress pad.
Most are really 3 pads together.
Wallyworld has several different size Fulton mattresses
that may work well, while not being expensive.
Especially if you are not leaving it open most of the time.
A few measurement and comparison shopping then
you should be good to go.
We are K&K for Kathy & Ken - Both retired.
She accounting, he Law Enforcement