Same camper, same truck.
What I have done is put a ROUGH piece of cedar 2 inch by 6 inch on its edge wedged tightly across at the front of the box. You need full dimension 2" by 6" for the thickness. When I lower the camper down, I start about 3/4 of an inch out from the top of the box, and then looking down I can clearly see the front bottom edge just tuck in behind the cedar 2 by 6.
Using a 2 by 6 like this also really helps to align the camper in the box. It's easy to see if you have one side out from the other and the camper in sideways slightly. It's really important for your mirrors to have it as close to perfectly placed as you can.
If you use the bumpers on the camper to prevent movement in the box, you will push out the top rail of your truck. Doing it this way the front of the camper is held about 7/16 of an inch away from the top rail.
PM me if you want any more hints on owning one of these campers.