Peel the green skin from the watermelon, leaving just the rind. Halve the watermelon, then scoop out the flesh, saving for another use; cut the watermelon rind into 2-by-1-inch pieces.
In a 4-qt. pot, bring 4 cups of water to a boil. Stir in the salt to dissolve and remove from the heat. Add the rind and let sit in the brine for 3 hours.
Remove the rind and transfer to a large bowl, along with the alum and 4 cups of water. Stir until the alum has dissolved and let soak for 2 hours.
In a piece of cheesecloth, tie together the cloves, peppercorns, cinnamon, and star anise into a sachet. In a 6-qt. saucepan, add the spice sachet, the sugar, the vinegars, and 4 cups of water. Bring to a boil, then reduce the heat and simmer the syrup for 20 minutes until aromatic and slightly thickened.
Meanwhile, remove the watermelon rinds from the liquid. Bring a large pot of water to a boil and blanch the rinds until tender, about 5 minutes. Transfer the rinds to the syrup and continue to simmer for another 30 minutes, until the rinds are translucent and suspended throughout the syrup. Remove from the heat and let sit, uncovered, overnight.
Meanwhile, submerge four 12-oz. canning jars, along with their lids and ring bands, in a large pot of boiling water and sterilize over high heat for 10 minutes. Transfer sterilized jars, lids, and bands to a clean dish towel. Fill each jar with rinds and syrup, leaving at least 1⁄4-inch of space at the top. Wipe jar rims with a clean dish towel, place lids on jars, and secure ring bands. Transfer filled jars to a canning rack; place rack in a pot of gently boiling water so that jars are submerged by at least 1-inch; let boil for 10 minutes. Transfer jars, set at least 1-inch apart, to a dish towel and let cool, undisturbed, for 24 hours.
To test that jars have properly sealed, unscrew bands and lift each jar by the edge of the lid; if the lid holds, the jar is sealed. If it loosens, jar is not fully sealed, and rind should be refrigerated and used within 2 weeks. Sealed jars will keep, in a cool, dark place, for up to a year.