I don't have my own hydraulic press (yet), but we do have one where I teach. I was able to purchase a square container kit, which included a 4" steel box, a square pusher, 6o durometer urethane pad, a liquid lubricant, a sheet of plastic to cut as needed for "intensifying", and several 1/4" deep acrylic squares to cut into dies. While it wasn't cheap it was definitely worth it. By containing the urethane within the box, it is forced to spread into the die. This yields a much deeper form than can be achieved without the container. People have made their own with a perfectly cut steel tube, welded to a flat sheet, filled with urethane. I haven't begun to investigate this option, because it is my understanding that working with the urethane in liquid form is highly toxic. I purchased the kit from Rio Grande. If anyone has experience with making their own, I hope you'll post about it.
I'll take some pictures with and without the box shortly. It makes such a dramatic difference that I can't imagine having a press without one, it is officially added to my wish list.