Amarine Made Boat Yacht Rectangle Opening Portlight Porthole 15-3/4" x 7-7/8" Replacement Window Port Hole - ABS Tempered Glass, Color: White, Black (Black)

I built my own truck camper (more like a cheap earth roamer though) and my plan was always to do the same. I have the slider in the pick up but I didn't find an acceptable hatch for the camper since it needed to be water tight from the outside yet open inside. Closest I could find was boat deck hatches similar to this Edit: as per your other comment about the wall thickness, I had to make window "trim" shims for all my windows because the walls were too thin as well. Might be necessary for your project as well.