Step 1: Materials
48 x 48 piece of 5/8" plywood
48 x 48 sheet of 24 ga. sheet metal
Step 2: Plywood base
Step 3: Poof! Chalkboard.
Luckily when I did this the humidity in my area was in the single digits and the temperature was low 90's. The paint dried and cured sufficiently within 2 days.
Once the paint was cured I dry fit some inexpensive wood base board for a nice frame.