My neighbor has three big cottonwoods on the fence line. Wonderfull shade trees, but murder on ac coils. This past Spring, I bought some aluminum window screen and magnets from HomeDepot, trimmed to fit, and stuck that thing around the unit. Now I just take the screen off, spray it clean, wash the dirt out of the unit, and put it all back on. Makes a tough job much easier.
I may make a frame to go around the AC unit and do something similar with the window screen. Can you post a pic of how yours looks?