Acebird, building hives 10 years now. A lot of the boxes here are CDX (Today, RTD), some solid wood. I get a lot of cull, and force the wood into shape. Still some of the boxes have big gaps in the corners, and the little bees fill it up.
Member, go to a house building site, and ask them if you can get some off-cuts. I use the SPF or SYP (It's generic wood. They mean, Spruce Pine Fir, and Southern Yellow Pine.) end cuts to make my side bars for the frames. Anything that will handle the weather, RTD, or Solid wood, longer than 20 inches. You can pull apart Pallets, they're good to practice on, being free, as long as you can find the wood stamped with HT and not treated wood. The HT means Heat Treated.
Pressure treated is a no-no. They used to use Copper Arsenate, which is toxic in any form, but now use borate "stuff," I think, and are more environmental friendly. If you go to the Big Orange or other Big Box stores, you can find wood that's twisted or damaged, off cuts in the section where they cut wood for you, and "Stickers," which hold the wood apart from other bundles that they stack it on. Look for the "Cull" pile in your local Building supply. That wood can be as much as 70% off. Ask if it's pressure treated.
You would want 3/4 inch thick lumber, the problem is the inside dimensions. NO SPACE LARGER THAN 3/8 INCH! Making boxes from thicker lumber is alright, but will have to be longer to accommodate the increase in outside dimensions. And makes things heavier. You would want to stick to standard sizes for trading frames and so on, and the boxes will not fit the same, so I would put the thicker wood on top so the water wouldn't run in the cracks.