Build a cargo box and let the dog lay on top of it or put his create on top , go the other way with it instead of reinventing the wheel, there are plenty of rear cargo box ideas out there that you can find one to suit, it will be much more efficient than building a create for the dog and trying to store stuff on top of the dog. I also i was thinking of opening up the backside of my cargo box so i can lay a seat down a get a full length rifle in the cargo box , my current drawer sizes stop me from being able to get anything in there . Go out to the back of the truck with your lr and take some measurements of how you want it to sit along with the rest of youre gear and then simply build a platform over it , put some tie down points in and strap the dog create in, you could also make a removable extension and use it for a sleeping platform in a pinch, again just search around and you'll find an idea to suit you, I've seen really simple ones than come out in 2 mins , but personally i rarely miss the 5~6 inches of height that my semi permanent box takes up back there .
