I use 2x6 when making slate crates. I like to frame around each slate with 2x6 and then use 2x4 for the uprights on both sides. With the slate sandwiched in between the 2x4s it is really close to the same thickness as the 2x6. I make a seperate crate for each slate so it is easy for the movers to use a handtruck on each one. Dont forget to mark the top of each piece so they dont set the crate upside down on the slate pins if you have those.
Good Luck and
dont fall into the trap of making a cheap crate and then be scratching your head on the other end wondering how they got broken!!
if you call me I can take a few pictures of some slates that are crated for shipping they way I do it, and send them to you.
Gordon
425-275-8255