Construct the lumber frame for the flatbed trailer according to your needs. Measure and cut the appropriate pieces of lumber to construct the frame. Use two identical pieces for the width of the frame, and two identical pieces for the length of the frame. Drill a hole in the end of each piece of lumber, then attach the four piece with four brackets and eight bolts.
Measure the inner length of the frame, then cut two lumber pieces to fit inside the frame. Position a piece of each lumber 6 inches from the edge of the frame and bolt them in place using two brackets and four bolts for each. This creates a wheel well for the trailer.
Measure the inner width of the frame, the space between the inner most pieces of lumber. Cut four slabs of lumber to fit this measurement. Attach the four pieces of lumber, width-wise across the frame. This gives the flatbed frame a ladder-like appearance, which reinforces the build.
Mark a location for the wheel axles, two-thirds the length of the frame from one end. Center the electric box cover over this mark so half of it extends toward the ground, past the width of the lumber. Drill two holes into the electric box cover while holding it against the marked lumber. Bolt the electric box cover to the lumber wall. Repeat this process to attach four electric box covers to the frame. Ensure you place the electric box cover on identically-placed marks on the lumber walls. This creates a carriage for the wheel axles.
Drill a hole in the lower portions of the electric box covers so the wheel and axles fit neatly in the wheel wells. Secure the wheels with an appropriate-sized nut and washer on each end of the axles.
Measure a piece of plywood to fit on top of the flatbed frame. Include measurements for the wheel dimensions and cut a rectangular space to allow the wheels freedom to spin. Lie the piece of plywood on top of the flatbed frame, and secure it with six screws on each long side, and three screws on the short sides of the frame.
Drill two holes into the bicycle tow arm at appropriate locations, then use bolts to attach it to the edge of the flatbed trailer furthest from the wheels. The trailer is ready to be attached to your bike.
