Fender washer on the top, fender washer on the bottom, nylock nut. At least that would get you home.
For my install kit I was given all the above (in 3/8"), with nicely forged shoulder eyebolts, along with an aluminum pad about 3.5 x 6", maybe 3/32" thick, to serve as an additional huge washer/stress relief on the bottom, pre-drilled with a 3/8 hole.
In my unexpert opinion, a 1/2" eyebolt with corresponding larger fender washer would be the same or maybe a bit better than 3/8" for stress on the bed's sheet metal.
You do want to be sure that the hook on the turnbuckle is in proper contact as deep as it should be to the eyebolt. If it doesn't fully contact they eye, the hook will tend to open when stressed.
Quick links come in handy in case of too-short turnbuckles. I used a set of 1/4" on the camper side.
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