Place a piece of plywood the size of the bicycle trailer beneath the bicycle trailer. Cut the plywood using your saw so that it covers the bottom of the trailer but cuts off where the wheels begin so as not to obstruct them.
Attach the plywood to the bottom of the frame of the trailer using a power drill and screws.
Saw three additional plywood pieces so you can place them on the inside and back vertical sides of your bicycle trailer to build your wagon's walls.
Join the walls inside the trailer with your power drill and screws.
Place one 1 x 2 inch piece of wood against each side of your bicycle trailer, on the left and right, laying on top of the walls, parallel to one another. The wood pieces should equal the length of the bicycle trailer.
Drill through the wood to join both pieces in place to the trailer's frame for your wagon's walls.
Put the wooden broom handle beneath each 1 x 2 piece of wood and attach them to make handles for your wagon. They should be long enough to protrude from the backside of the wagon when it is closed.