Update. Since my right elbow is killing me, I decided to make an improvement to reduce the weight of these southern yellow pine pieces. Negligible loss of strength.
I failed to mention that I arrived at the 6 7/8" rip dimension by actually putting a short length of plywood onto the wheel wells to see exactly how high off the bed surface it was.
Here's my suggestion, which I just finished yesterday...I used #2 Southern Yellow Pine lumber, super strong and water won't bother it at all. Using one 2x8, I ripped it to 6 7/8" - more on this later. I have a spray in bedliner and cut the longest piece to 54 3/16". It could be 54 1/8", but 54...
My '23 is doing this as well, and the truck has not been driven in two days, so it's not the exhaust cooling. I think it's communicating with someone or someTHING in the Andromeda Galaxy. Probably discussing why Ford doesn't ditch all-electric vehicles and stick to hybrids so they stop losing...