Bunk Bed Construction

I'm building two bunk beds in my cargo trailer, and I've been going back and forth on how to build them.
Goals:
Sleep an adult male if needed (let's say 300 lb capacity) (they will mostly be used by my kids, but I do get out with the guys and wouldn't mind the option of them sleeping in them if needed)
Keep it lightweight (ish)
My bunks are going to be 30"x80" (one will span the width of the trailer, and other will be running more lengthwise.
My current plan is to use 2x4s and run a rail on each side capped at the ends with 2x4s, and use 1/2" plywood on top, but I wonder if I should be doing a third 2x4 down the middle ish, or if 2x4 is even strong enough. I also wasn't sure if I should be doing some cross pieces.
Let me know if I didn't include any info, or whatever.
Goals:
Sleep an adult male if needed (let's say 300 lb capacity) (they will mostly be used by my kids, but I do get out with the guys and wouldn't mind the option of them sleeping in them if needed)
Keep it lightweight (ish)
My bunks are going to be 30"x80" (one will span the width of the trailer, and other will be running more lengthwise.
My current plan is to use 2x4s and run a rail on each side capped at the ends with 2x4s, and use 1/2" plywood on top, but I wonder if I should be doing a third 2x4 down the middle ish, or if 2x4 is even strong enough. I also wasn't sure if I should be doing some cross pieces.
Let me know if I didn't include any info, or whatever.