What's the best way to prepare this ahead of time? I'm planning to make this 48 hours in advance, and I was curious if it would be better to freez...

Yes, freeze but do not refrigerate. Watermelon goes bad very quickly.

There's a comment on the recipe that someone made it & froze it in zipper bags. I think it would taste fresher if you froze it, rather than refrigerating for 2 days. With the gin in it, it may not really freeze hard
at all. It might be more like a gin watermelon slushie...and how could that be bad?