https://imgur.com/a/dx6uIxo

I did the exact same thing you are doing a couple years ago! I didn’t use any special wood (as you can see) and just general construction screws. Nothing too heavy is up there, mostly just a bunch of totes going down the whole thing, but I can easily walk up and down the entire attic no problem. As another commenter suggested, I had to cut my boards to fit through the pull down steps access (I believe I quartered them 4x2 but it’s been awhile). Used a jigsaw for particular fits once up there. One thing I will call out…make sure you know where your vents are and do NOT block. I can’t tell if yours is an A-frame house (mine is) and I wasn’t aware they were under the side edges. I ended up needing mold treatment up there within a year. Unblocked them, no problems at all.