I added one to the back of my GMC.
I bought from a marine surplus store a front deck hatch for a boat. Watertight with a 1/4" smoked plexiglas type lid.
It had all the seals and the hatch can be held open by an adjustable arm or flipped completely out of the way.
It had a minor scratch on the cover, but for $50 vs $500 new it was a deal. Very heavy duty.
You will need to frame around the opening to compensate for the hole you cut.
Last, but not least...Give it the test in the store to make sure it has enough clearence (width).