If you want to spend your time constantly fixing leaks, sure. Even if you never take it out in the rain, it'll be annoying when it leaks if you ever wash it.
You'll also probably need to do some serious roof reinforcement to keep the body from flexing and affecting whatever material you stick in there. That probably means more steel at the roof level, plus the weight of whatever the transparent material weighs, and an even higher center of gravity than it has now.
I say no way unless you're trying to come up with an excuse to sell/scrap the vehicle.
Rarely do roof mods hold up, even on street-only vehicles, or even show-only vehicles. I've seen many professionally-done roof mods like ragtops and panoramic roofs, and sure, they look great at first, but the vehicle winds up being too impractical and too much hassle to own and keep fixing. The flexing an off-road vehicle goes through would almost guarantee issues that would only get worse with time, especially if you do it with a metal L-frame and something like plexi inside it.
If you wanted to do it "right", I would say you'd want to grab the entire panoramic roof frame and glass assembly out of a factory vehicle, or even cut the roof around it so you're just welding the roof on and doing bodywork.
The Ford Flex has a roof that would work:
Or you could get one from a Land Rover if you wanted a larger one:
If safety is your primary concern, I would leave the roof alone. The Explorer body has shown time and time again it, by itself, is strong enough to stay intact in a rollover, even in cases where the vehicle flips and rolls many times. Even with a cage, a strong roof and body makes for an excellent first line of defense. I would say no to even slightly compromising that level of rollover protection and safety for something so cosmetic.
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