Thanks for all the comments. I would be fine with them roosting on the front lip as well as the real roost, so that may be something I design for with a "thicker" lip when framing the poop board. Maybe even account for front roosting by placing the "real" roost a little closer to the back wall. Just by an inch or two so they have that much less chance of pooping on each other. I don't really mind poop on the back wall since at this point I plan to design it so that I can somewhat easily clean the back. Vinyl back wall or something of that sort. But avoiding the need to clean the back at all is obviously going to be preferable.
I was sketching out different orientations for the bars and nothing seemed to net me much extra usage of the space. Right now I am planning a 24 inch by 60 inch poop board. Sketched out various things like using a "H" shape with two 2 foot bars running front to back, (one foot in from each side), with a 3 foot bar running perpendicular in the space between. That technically gets me 7 linear feet of roost with at least 12 inches between any edge or bar. But the spots close to the intersections of the "H" would not be usable. Either a chicken on it facing one way, or facing 90 degrees/the other way. So probably back to the same usable space as just running a single 5 foot roost down the long end of the board, 12 inches from both the front and back edges.
After all the mental gymnastics, I might just ditch the current plan and custom build something more to my liking. I could go wider than 5 feet by 2 feet in the coop, but that is what the dimensions are of the table I am trying to repurpose for nest boxes and poop board/roosts. If I can avoid having to make a larger "topper" for the table, I will. But I guess I could always toss a cut down 4 by 8 foot sheet of plywood on top of the table. I have about a max of about 6 feet of width to work with for the poop board. Chicken math hits often, but the plan for this is to house about 6 or 7 chickens. Hence why my 5 foot table starts to worry me with only 5 feet of roost.