Cattle panels seem to be a standard size of  16 feet, x 4 feet.  50 inches is just 2 inches over 4 foot. 
I do not know how far the 2 foot knee wall is away from the coop wall. That would  make the height of interior  vary on the distance.  The closer the wall, the higher the resulting arch. 
Not sure what your coop is constructed of, but if it is wood, and heavy enough, than these U nails should hold panels in place. 
		
		
	
	
Once you have all your panels  up and  constructed, you will also need to secure/overlap  at least the bottom 2 feet  with ether chicken wire , or HC,  to keep your chickens from walking out thru the openings.  6" x 8"  is large enough for many chickens to squeeze thru. 
Your run is secure to  a minimum   for most daytime  purposes.   It is mainly to keep your chickens in. 
THERE ARE WAYS TO MAKE IT MORE SECURE, IF YOU WOULD NEED IT TO BE.

Hope this answers some of your questions. 
 WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 
