Sheet of 2" foam.
Or
Some 2x4's flat on their sides with a sheet of 1/2" plywood on top.
Etc.
Lots of ways to skin that cat.
Just some advise:
1. Make it 4 inches wider on either side to give you a nice landing zone. Don't make it "just fit." You are not building a piano. Conversely don't make it too tight length wise; if you have a 80 inch bed, cut it off at 75 inches or even shorter given rule #4.
2. Paint it using an exterior gray paint or a "deck paint" so it does not wick water or rot.
3. Drive up a a curb or off road (close to home, within a mile) to make sure your camper does not slip off.
4. Make a block by the tailgate so your platform does not decide to escape. A 2x4 can usually be cut to fit the width about 6-8 inches before the tailgate bump-outs.
5. Measure 10 times, and be willing to rebuild again; don't use poly coated sinkers "stinkers" or drywall screws, use "construction screws" made in USA or Europe, not East Asia. Don't use screws from countries that have outstanding debt in the Euro-zone. PIGS.
6. Use a cheap bed liner to protect your truck bed. Cheap, don't spend more than $40 on this bed liner it is going to be trashed soon.
Hope this helps, if I have a point wrong let me know.