I built a 1/2 side sleep platform.
25" wide. 42" long. The front legs are an inch and a half longer than the rear legs to fill in the drop space upfront and be level. Total height is 11.5" throw an exped mattress in as needed. Decent amount of storage space underneath as well.
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Might be a bit late for you, but for anyone else wanting the same setup... Check out Roll-a-cot cots: Green Polyester Longhorn Roll-a-Cot® (79"L x 32"W x 15"H) with sleeve for your air mattress - https://www.camptime.com/collections/cots/products/green-longhorn-lw-roll-a-cot-79x32x15
Fantastic cots with a very nice structural design. No middle support/leg, meaning you can put this right overtop of the wheel well in the cargo area.