While a reasonable attempt for a young Pony I am fairly sure dehydrating fruit works better with much thinner, and more even-thicknessed slices. Watermelon is almost certainly especially challenging due to the quantity of... well, water to be removed and its brittleness combined with lack of strength.
Also, I THINK one leaves the oven door open a crack for dehydration?
Again, not bad for a young Pony. Gotta make a lot of mistakes when pushing your envelope!