Plans to build a bed, 6" high for my van. 28" x 81"
Don't know if this will work for you but they have some very good ideas:
Perhaps this will give you some ideas. https://www.hometalk.com/43883849/organize-your-camper
I looked up the dimensions for mattresses and found even a twin mattress is wider than 28". Had you thought about getting a piece of thick upholstery foam cut to your dimensions? You could put it in a washable zippered cover. What type of base were you thinking? You wouldn't want too much added weight. Good luck and stay safe!
P.S. here's a link for you:
Use old Pallets. Cut ro suit as a frame. Screw or nail together. Overlay with MDF or Plywood then , Add a mattress or slab of fire retardant foam. Sweet dreams.
here are more ideas
Here are some directions from Pinterest https://www.pinterest.com/pin/213498838574198133
Do you want it to hinge up out of the way? A 2x6 frame and plywood base should give you about what you want.
something like this? https://www.charlestoncrafted.com/how-to-build-a-platform-bed-for-50/
Why not just put a mattress on the floor in the van
I built one for a friend using plywood top with 2x4 frame and legs. Built it high enough for him to slide plastic baskets under it for storage. He really likes it.