Depending on how many dogs you are looking to haul it is pretty easy to make one with a couple sheets of plywood and some 2x2's. I've built a few that were 48x48 x24" give or take with two compartments and I could haul 6 dogs pretty easy as long as we were not going across the country. If you measure correctly you will have little to no waste. Don't skimp on doors, if you just cut squares out of the plywood and hinge them you'll regret it, take the time to build heavy doors. Also I put 3/4" plywood on the top and bottom, seems like with a box in the truck you will always be throwing stuff on top of it or someone will be sitting on it etc, adds some weight but with a heavy coat of fence paint inside and out it will outlast the dogs inside. Cut some 6"x42" slots in the side and hinge them and you will have plenty of ventilation that you can close in the winter and add some eyebolts so you can ALWAYS strap it down somehow.....ask me how I know.
The "dog box" has become sort of a status symbol for the modern day houndsmen and prices and designs are getting more crazy than I care to spend. Lots of 50/50 houndsmen out on the landscape these days....$50,0000 hunting truck, hauling $50 dogs, I prefer to put my money elsewhere in the operation.