Sigil. calibre (yes, with a lower-case first letter.) There’s even one called Hamster.
If you’ve ever searched for a tool to convert your manuscript to an ebook, you’ve come perilously close to drowning in geekness.
Stop. Step back. Put down the chainsaw; you don’t need one to make a toothpick. Especially if you already have a box of toothpicks.
Everything I say in the rest of this post will have exceptions. I’ll mention a few at the end, but if your manuscript doesn’t fall into the exceptions, don’t go to exceptional lengths to get the job done.