I've made several boxes and lids of plywood and they seem to hold up fine. Stick with real exterior ply, no osb, mdf, or advantec. Copper Napth Is found near the deck preservatives and sealers, in a can labeled term-in-8. Around here it can be found at home depot, lowes, and Ace. Smells like liquid death and takes a long time to dry but it will protect wood in contact with the ground. The product I mentioned is 25%Cu-napth and I've used it cut with terpentine and small bit of boiled linseed oil to 2.5%active ingredient, brushed only on the outside of the boxes and dried for 1 week before priming. Do it (application and drying) outdoors. I cut it to 2.5% because I've read that it's still effective and that conc. was recommended for beehives or above ground applications under paint. Sorry I don't remember where I read that. I've seen the program on RFD, that guy sloshed the green stuff all over (and in) the box. Based on the fact that all edges inside my hives seem well chewed/smooted by the bees, I've limited my trials with the copper applications to the exterior of the boxes. Does it really make a difference in box servicability over time? I don't know yet.