I almost hate to admit this, but I used pressure-treated wood for my original framed beds 20 years ago and the same when I rebuilt them 2 years ago.
But I really did a lot of research, and I became convinced that the chemicals in new pressure-treated boards just don't leach into the soil to any significant degree. If they did, they just wouldn't last long, would they?
I'm 66. If I was 20, I might worry about it more and use cinder blocks or Trex lumber.
The construction was straightforward; 8' 2x10s two levels high 3' and 4' widths. Easy to build.
And then I completely enclosed it all in chicken wire on PVC pipe. Keeps the critters out but lets the pollinators in.
The door was the hardest part. I set 4"x4" posts down 2' deep and screwed in angle braces. To this day it keeps shifting slightly. I adjust the latch to work one month and it is loose again the next. I suppose it will all settle in someday, but for now, I have to keep a cinder block in front of the door to keep the groundhog from nosing in,
He can't dig under the fence; I extended the chicken wire 2' out onto the lawn at the bottom all the way around. I see him sometimes pulling at the fence. It does him no good, but I really need to kill that big rat (he eats my wildflowers when he can't get at my garden).
It sure baffles the squirrels. I haven't had one find a way inside yet. I even put dabs of peanut butter of some of the frames and they never got touched. And no squirrel will pass up peanut butter.
The enclosure still needs work. I have nylon pull-ties every 2' holding the strips of chicken wire together, but I need to "sew" it with thin galvanized wire to really secure it.
If you think I'm overdoing it, you haven't had squirrels eating 3 successive plantings of corn or possums taking bites out of every tomato.
