So, I decided it was time to make one. I gathered up some old clothes and my measuring tape and went to it.
One trick I used to put the costume on: in order to keep my usually-docile guinea pig from freaking out, I first just placed the costume on top of Annabelle while she was on my lap, and letting it stay there, as if it were a blanket. Then, before she could protest, I attached the one going under her belly. After a few more snuggles, I then (just as quickly, if not quicker) attached the one around her neck.