This instructable will show you how to create a deck plate and sides for your Harbor Freight utility trailer frame. While this example is for the model/item no. 42708, the techniques may be applied to all models, with appropriate dimensional revisions.
1/4" Diamond plate x 41" x 48"
(12) 1" x 4" x 8' pressure treated boards
(40) 1/4" x 2" galvanized carriage bolts
(8) 1/4" x 1" galvanized lag screws
(8) 1/4" x 1-1/2" galvanized lag screws
(56) 1/4" galvanized washers
(48) 1/4" galvanized nuts
Large bar clamp
Step 1: Deck
Attach the deck plate using the 6 stock hex head bolts along the frame rails. You may also add additional bolts in the center of the end rails and center cross member.
Step 2: Stakes
Before cutting your boards to length, cut a small bit off one end of each board. This will give you a clean edge to measure and cut from.
Cut eight stakes to length. To maintain the same dimension, you may want to clamp a scrap board to your table to act as a stop.
Place the stakes on top of your stacked boards for reference (as shown in the fifth photo below) and mark the center of each board on the stakes.
Drill holes for the carriage bolts in the stakes. If using a drill press, you can set up a fence, so that all of your holes are in the middle of the boards.
Place the end of each stake in the side rail pockets. If the stake does not fit all the way into the slot because of bolts in the slots, simply notch the stake(s) with a bandsaw.
Mark the location of the mounting hole with a pencil and drill a pilot hole in the stake. Attach with a washer and 1/4" x 1" lag screw, leaving a loose fit. (This will allow you to square it up in the next step.)
Step 3: Side panels
If the hex head bolts in the deck prevent the side boards from making contact with the deck, create the appropriate notches with the miter saw.
Stack 5 boards up along one side, and clamp them between the vertical stakes, and scrap lumber. Using a carpenter square, straighten the stakes. (This is why we left them loose in the previous step.
Using the holes in the stakes as guides, match drill the side boards.
Fasten with the carriage bolts, washers, and nuts.
Step 4: End panels
Measure the inside of one side to the inside of the other side.
Repeating the techniques from the previous step, cut and attach 5 boards to the front and 5 boards to the rear.
Step 5: Corners
Using a piece of scrap, line up the end and side panels, and clamp in place.
Place a corner brace inside, drill pilot holes, and attach with a washer and 1/4" x 1-1/2" lag screw on each side.
Tighten all the lag screws at the bottom of each stake, and you're ready to haul.
Since each side is only held by 2 lag bolts at the bottom, and 2 at the top, you can take off any or all sides with a socket wrench in about a minute.