I can't quite see if you have the dimensions on your drawing, so I'm not too sure what the feasibility is. I would suggest make it with enough room for you to easily clean it. As for attaching it to the dog run, you can make tunnels out of wood/pcv and chicken wire/fencing. The could be long or short depending on how far away the pen is. Or if you want to move it to give them new areas. I haven't tried them yet, but if you do a Google image search for 'chicken tunnel' they look promising.