In my boat I have the standard vinyl covered 3" foam on the top...... under that I bought 2 foam single beds from Clarke, covers and all . I took the originals in as a template and they pulled out the foam and trimmed them for me. My wife then ran up the covers to the new shape. We reverse them  at night and sleep on the cotton  covers with the vinyl ones on the bottom. Cheaper then buying the foam  at Clarke on its own. 
Now ..if you want cheap... 
 
Another angle ...people often throw out lounges with torn covers . The foam is often good quality. You can trim and glue foam into large pieces with contact. I've done it in my old Kombi.
To clean, hose it down and shampoo with detergent. I once bought an old caravan with an excellent 5 " foam bed we washed it but it held the water for days  

 solution ...put large piece of ply on it and drove the car onto it. Squeezed it dry  

 not kidding it works.