DW, we got it from our dealer when we bought the Precept. Think it works fine. I'm 5' 10" and fit just fine without it, but we do use it when we winter in Texas. Their available on line from $ 65-$ 75. I've seen some people that made there own. they used 1.25 density foam, but you could use whatever your local fabric store has in stock. Tell them that you want to make a piece of foam thatís 60″ long (or as wide as your bed), 7″ tall (or as tall as your mattress) and 6″ wide (or however long you want your bed extension to be). It fits at the top of your bed so no need to support anything. We store it away in the closet when traveling because we can't close our slide with it in place. I hope this helps.
Me, Mrs., and Maggie
2015 Precept 31 UL
2015 Equinox, Blue Ox Avanta LX