I made a flat aluminum roof out of 6061 aluminum. It was very durable and worked great. The problem was when a branch caught the visor or hit the top it would ring your ears. It was pretty loud.
I now have a top made of 1/2" HDPE. I made it a little extra wide and it works great as a rub rail on the top of the roll cage. Its quieter and absorbs the impact of branches much better. Both options are heavy enough to stand on, and another bonus is you can screw a dome light to the roof and not have to worry about fasteners on top or making holes through the roof as I did with aluminum. I really like it and would recommend the HDPE route.