I brought a suitcase (1 size up from a carry on). I wish I had done the milk crate/hand cart method. The suitcase was so heavy, it would have been nice to have it broken into the smaller crates. They let me keep the suitcase next to me, but I happened to be seated at the end of a row. Otherwise I would have probably had to stack things next to me.
I've said this a few times over the years, but ignore people from outside the Environmental discipline when they tell you to only bring a few references that you really need and that you used while studying. The Environmental exam has roughly 50 qualitative questions and is an extremely broad exam - I used about 20 references while studying and I honestly used nearly all of them on the exam too. I had time to look things up in a couple different books if necessary and having lots of sources for somewhat obscure regulatory questions may have made the difference between passing and failing.