2- 4x8 x 3/4" AC Plywood 23.00 each/46.00 ( you'll find it at the box store labeled 23/32 x 4 x 8 Fir Sanded Plywood)
Lowes has it for 23.00
( AC plywood is smooth on one side but since this piece is going to be covered by masonite a cheaper piece could work)
1-4x8 x 3/16" masonite $15.50 ( the plans call for 1/4" masonite but HD and Lowes usually only stock 1/8" and 3/16")
5- 2x4x8 2.95 each/14.75
1- 10"x48" x 3/16" Metal sheet- about 20-25.00 ** Update Local steel shop will charge 31.40 but steel shops have varying prices depending on area**
( 10x48 will have to be cut from a bigger piece)