You will have to cut it a little beforehand, but it will make those final few cuts a lot faster (and more fun).
Step 1: Materials
4 Feet of PVC pipe (I used 1 1/4 inch, but other thicknesses may work as well)
4 Right angle elbow pieces
Roughly 8 Feet of thin metal wire
A pair of needle nose pliers
A power drill
Step 2: Cut the Pipe
I would suggest 1ft lengths.
On my prototype I used two 1ft lengths and two 1 1/2ft lengths.
I found that this was not necessary, because only 1ft of the 1 1/2ft length was used.
Don't forget to sand down the ends of the pipe so that it will fit into the connector properly.
Step 3: Attach the Sections
This will be the frame for your watermelon slicer.
Step 4: Drill the Holes
Drill a hole through the pipe at 1 inch intervals on two opposite sides of the frame. I would suggest arranging it so that you are threading the wire through a thin hole, and making a larger hole for it to thread out of.
Step 5: Thread the Wire
Thread it back and forth, pulling it as tight as you can (I used a pair of pliers to get a better grip). This will leave you with series of parallel wires (it will look something like a harp). Tie off the wire at the other end.
Step 6: Completion
Note that I had already chopped the watermelon in half before attempting to cut it with the slicer.