I just purchased my ProMaster and will start doing the conversion. The van came with a Rear seat (3 passengers) that converts into a bed, like a futon. The dimensions are 48" x 57". I really don't like the color, but it is in pristine condition. The previous owner bought it in 2018 and he says...

Innovative 2-person seat converts into a 6'1" bed that spans swivelled front seats for Adventure Van and RV conversions. Sits against the driver's side wall, SLIDEs right to center up to 11", then SHIFTs into a bed. Made in Italy by the OEM manufacturer FASP. Crash tested and totally awesome...