Bill Tindall, E.Tn.
Use dry sticks, cover top with rain shedding something which could be as simple as two layers of low grade lumber (I use roofing tin) and ensure the stack is in a location that gets excellent ventilation. The dry sticks and good air flow are most critical.
I have never seen the sides of a stack covered in any way. In their quest to keep rain out people often err by not providing good air flow through the pile.
Maple molds to the core in 2 days in the summer if you don't get the surface dry quickly. Mills that don't have mold treatment capability often don't risk sawing maple in the summer, of if the do they get it on sticks the same day it is sawed.