I've purchased several dual sport bikes and one of the first things I add is a rear rack. The XT250 I recently purchased didn't have one, but rather purchase one I decided to see if I could come up with a less expensive option.
I've found that plastic cutting boards can be used in a variety of projects. The plastic used for the boards is strong, easy to cut and shape, very stable, and obviously resistant to any corrosion. I studied the rear of the bike to come up with a plan and then headed off to Walmart to see what I could find. The board I chose as about $8.00.
I always carry the bag in the pictures. It holds tools including everything needed to patch a tube anywhere you might be.
While I only drilled two holes in mine, additional holes or slots can be cut to make this more "rack-like". I added a brace that I shaped from a piece of mild steel I had on hand. The brace attaches to the bolts at the rear of the grab handles using longer bolts, spacers, and additional nuts. Two machine screws secure the board to the fender and the underlying brace.