What you need.
1. Foam 4x8 sheet, the thicker the better - Home center
2. Knife with a long blade
3. Latex Paint - Craft store
4. Spray glue - Test to make sure it doesnt dissolve the foam - craft store
5. Wooden dowel - Craft store or home center
6. Plaster - Home center
Step 1: Cut n Glue
Now glue 2 2x2 pieces together with spray glue then repeat. Then glue 2 2x6 pieces together and repeat.
Step 2: Reinforce the corners
Then grab your 2x2 pieces and line it up against the edge of the 2x6 piece. Now push the dowels slightly into the 2x6 to mark where you need to drill.
Drill out these marks, add glue then join the 2x2 with the 2x6. Repeat for all 4 corners.
Step 3: Plaster
After you put the plaster on sand it down a bit so it is even with the foam.