I made a train table for my boys last week. If you expected me to jump right into how to make plyo boxes, you were way off. The beauty of making a train table for my boys is that we saved $150-250 by not buying one. The other beautiful thing about it was extra wood. 
All I needed was a piece of 3/4" plywood, screws and glue (I already had the screws and glue), and I was set. Because I used part of the plywood, I was limited as to how big I could make my box. I ended up making a box that is 18" x 20" x 24", which is great for my needs as a trainer. If you have a full sheet you can make bigger boxes, or you can make them much smaller to fit your needs.
However you end up making it, you end up with a plyo box that has three heights and in order to buy three different boxes anywhere, you will spend several hundred dollars. This is free, or at worst around $20-$30. Here's how I cut my left over 5'x4' piece of 3/4" plywood (actually Lowe's made the first few cuts...their saw is awesome! If you're there, ask them to cut your wood at 18", 18.5" and 18.5" across the 4' width of the board).
2 - 18"x24"
2 - 18.5"x24"
2 - 18"5x16.5"
Put them all together using wood glue and 1 1/4" screws and start jumping.
If you're looking for a bigger box, here are the cuts you need for a box that is L= 30″ x W=20″ x H=24″
2x 30″ x 22.5″
2x 30″ x 20″
2x 18.5″ x 22.5″
