Camping bed extender?

hi friends,

I plan on taking my crew max tundra with a camper shell camping soon! Problem is the bed is roughly 5'8" long and I am a little taller than that. Im asking for your photos if you may kindly share or ideas. I plan to sleep in my bed with the tailgate down, and camper shell window down/ closed position. To illustrate I plan on building a rectangle/ box shape camping bed extender so I can enclose the open space between the open tail gate, etc.

I am freak of nature and I like things secure when I sleep, or I would force my step brother to pull fire guard duties. Ideas or pics would be great, I plan on buying a good qualit piece of plywood and build from there.