A while ago, I tried out to make ponds with specific shapes (although I had to kinda give up the idea because of space problems), so maybe I can help you with some tips I used back then. If you want to make a Eevee shaped pond (which is a very cute idea btw!), I think it's better to make a bigger pond instead of a small one, especially if you care for details and such (like for the ears and face in general). So make sure you find a good spot on your island where the pond would fit without maybe blocking any houses, ways etc. later. What I did was first making a sort of rough outline with a dirt path. Once I liked the way the outline came out, I filled it out part for part with water. I also made then the fine details, like round up the edges etc. It may takes some tries to get the shape as well as the size you want right, but with some patience, it should work. Good luck!