I've not tried growing melons in clay, but my usual prescription for clay gardening is raised beds with lots of compost well mixed in the top several inches of soil and well mulched. I like the beds to be 4 to 5' wide raised at least a foot after settling. It's worked well for me with blueberries and raspberries- 2 non clay loving species. In areas that need heat I'd use some extra compost instead of mulch on top covered with black plastic.