If I was doing it I’d probably do something similar to this…
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#4042 Rear Flip-Up Hatch
www.hardbody4x4.com
The pictures are from Aqualu’s site. The hinges they use would likely be easier to source than original Fj40 hinges. Alternatively, you could keep your one piece door and build a custom tailgate. I’ve always thought the one big door would be convenient. I do, however, like having a tailgate/tire carrier and a hatch. Unfortunately, I’ve never quite got to designing weatherstripping/hardware to seal the hatch to the tailgate. Over the last 20+ years I’ve had several different ideas. Hopefully 2025 will be the be the year to fix all the weatherstripping on my 40. Even the foam in the heater is worn out. After decades of a breezy 40, I don’t know what I’d do if it was no longer a wind tunnel inside my 40.