Drop verticals from the ceiling to support the roosts in the middle. On the ends I put a small section of 2x4 on the wall, set the ends of the roosts on those, drilled a hole in them and dropped large nails in those holes to hold the roost steady. I used tree limbs instead of sawn lumber for roosts. You could screw them in place instead of using removable nails if you want but that makes them harder to remove.
You can use an 8' section and a 6' section of 2x3 to get the length of your roost. There is nothing that says they have to be one continuous piece of wood. If you do want it to be in one continuous piece, use that 2' cutoff as a splice plate. Center the 2' piece over the joint and use screws to join them into one long piece. Be careful handling that long piece so it doesn't break. Once in place and supported it should be OK.