My coop is 6'x3' so a bit smaller than yours. I had the same problem with fully opening doors and nothing to attach the roosts to. Maybe you could build a new configuration with (2) 4' roosts and a larger poop board underneath similar to mine. But you may have to add a center support to make it work.
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Not sure about adding the roost(s) to go the full 6' length of the coop. Without being there, I don't see how you could make poop board underneath and still leave enough space for them to jump up and down... maybe it would work.
It looks like you have 3 nest boxes, I see 2 dividers. I don't know if there's a solid surface behind the curtains, if not, you can add vertical supports in front of those dividers to attach roosts, like so:
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The orange would be the actual roost closest to the open doors, the darker color is the "step." It may have to be lower than I drew it. But 6" still might not be long enough to fit 8 birds after they fully mature, and lower pecking order birds will probably be pushed off and roost on the lower bar.
Another option would be to make both the same height (which I prefer). They don't have to be nearly as high as you have now, just higher than the nest bottom. If a poop board doesn't fit that way because of the nests, you could always just fill with coop floor with sand/Sweet PDZ mixture and scoop it out regularly like cat litter. Resting the roost bars in something like a closet rod holder or fence bracket will make them easily removable for cleaning.
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Oh, and if you really do only have 2 nests (one divider), you could still add the vertical supports but take out all nest wall dividers to leave one long community box. Some chickens really like it that way.