There’s a guy in Pulaski, VA that always drives around and he has a flatbed truck made out of Lumber. He did a good job jointing the pieces and staining them. As far as I know it’s legit in VA and nobody has stopped him.
He has the brake lights mounted on the bottom frame which is visible in the current set up, but they are still high enough off the ground to be legal. Check the ordinances for your state when doing your lights, I know some states are pretty strict on how low or high the lights can be off the ground and the bumper, shouldn’t be a problem and unlikely somebody is going to pull you over for it… but always worth avoid that 75 dollar ticket to the court house.
Are you handy with framing and trim? If so you can build some pretty decent looking housing for the lights using smaller lumber and staining it to match. As far as removable slats, I’d say reinforce them with some square steel tube (pick up some at your local Lowes. Attach to sidewall/slats and create a mating end in the truck bed and some other secondary guarantee that they wont rip out the back and lodge into someone’s windshield (e.g some chain, hangars).
The other option is to get a more spiffy looking truck bed, go to your local junk yard and look for the bed of the truck, or if you know someone that welds you might be able to bribe them with some good beer to help out. The one other aspect to consider is this… whenever you significantly modify your car from it’s original manufactor’s design you will have to register it with a rebuilt license (small nominal fee, mostly red tape), have it inspected, and a new VIN assigned. I’m not a lawyer or someone qualified enough to venture a guess on whether this would require a “rebuilt/kit/conversion” license but it might. I’d make my first stop talking with a body garage for some details. Food for thought.