First of all, people come to us for suggestions on how to make things edible or even more edible. Sounds to us like you need to track down a service like, "OBiologist" or "OFormaldehyde Man." Second, we know of no one who preserves watermelons, either whole or in part. Pickles made with watermelon rinds are a popular if somewhat old-fashioned preserve. We don’t think the flesh of a watermelon has enough structure to stand up to traditional methods of preserving.
Third, of course we have no idea what sorts of vile experiments you intend to subject your watermelons to, but won’t the addition of sugar, salt, vinegar, alcohol and/or spices change the nature of the watermelons enough to throw off your results? Preserving often takes a fruit and transforms it into something else entirely. No matter, there is a method used to preserve whole fruits, although we’ve never heard it used for watermelons and you’ll have to modify it somewhat or you won’t save a cent for your club - drown it in brandy or some other alcohol.
Some fruits are cooked first, but you wouldn’t do that with watermelon. Prick the rind through to the flesh with a skewer in a few dozen places, then put the watermelon in a container that you’ve somehow manage to sterilized. Pour in enough of the preserving liquid to cover it completely (you’ll have to weigh it down with a heavy platter or something else that can be sterilized, because watermelons float). Devise some kind of airtight seal, and store it in a cool, dark place. As to what you use as a preservative, you really should check with a scientist who may know what you can get in bulk for a reasonable price. We know of no such sources of brandy.
If you freeze the watermelons, the tender flesh inside will break down, but, again, as we have no idea what you're up to, perhaps that would be acceptable. Of course that leads to the issue of finding space to freeze multiple watermelons, which could also involve substantial costs.
For those who still think of this as a cooking site, here is a traditional way to pickle watermelon rind.