Materials: crate, plywood (half inch thick), foam, fabric, adhesive spray, staples (8mm or half inch) and staple gun
Step 1: Take your crate to Lowe's or Home Depot. They will cut the plywood for you for free. Measurements vary depending on crate size/shape. You want the plywood to sit on the lip of the crate. They can measure and cut to fit.
Step 2: Measure and cut your foam (the cheapest I found was egg crate foam for a twin bed) to match the plywood. I used scissors and they cut through the foam fine.
Step 3: Spray the plywood with the adhesive spray. Place the foam on it and press down. Any hanging over, I cut off.
Step 4: Lay out your fabric and place the foam/plywood on the fabric. Measure and cut around leaving 2-3 inches extra to fold over to staple. The corners you can fold like wrapping a present. Pull the fabric tight so it is tight, then staple. I used 5-6 staples on each side.
Step 5: Place it on the lip of your crate, and you are finished!