Pixel didn’t need anything, because she had a laparoscopic ovariectomy. (which I wouldn’t recommend for a small Havanese after our experience) But she only had a couple of internal stitches, so she had no need for a cone or onesie.
Panda wore a cone and had no problem with it at all. But she is very much a go-with-the-flow girl, and she was 6 years old when she was spayed. I have a breeder friend who uses lycra surgical suits with her girls when she has them spayed and likes them very much.