I built my own flat bed for my F150 using southern yellow pine. Watch how I did it.

Materials list:

Southern Yellow Pine .
12 - 2x6x8' (for bed floor and skirts)
2 - 2x6x12' (for taillight assembly rail - remainder used to make headache rack)

Treated Lumber
4 - 6x6x8' . (cross members)
4 - 1x6x8' . (side rails)
1 - 2x4x12' (side rail supports)

8 - 8" galvanized 1/2" carriage bolts w/ washers and nuts . (attach 6x6 crossmembers to frame)
22 - 6" galvanized 3/8" lag bolts w/washers (attach skirts to side of bed, and taillight assembly)
2 - 8" galvanized 3/8" lag bolts w/ washers (to stregthen headache rack to bed)
2lbs 4" deck screws
1 lb 2 1/2" deck screws

Taillights to taste