Create three "caps" using one of the methods shown in the image.

Cut one piece of OSB to size, then reinforce with 2 long 2x4s laying flat on the outer edges. The 2x4 pieces should be 3' 9 1/2" long. Fill in blocks in between the long 2x4s. Screw in two more 2x4, 15" long, on top of the long edge 2x4s - to create effective handles. The project past the edge and come to rest on the foundation forms.

Alternative method is to layer up three sheets of 1/2" OSB or plywood.