To truly, successfully eroticize the monstrous you must be brave enough to make the object in question actually ugly. At the heart of the fantasy is a person whose very form is so profoundly unacceptable that there is significant social stigma attendant upon finding them attractive - a person who feels deeply their own repulsiveness, who expects universal rejection for their appearance - and, crucially, another person who treats them as if they are beautiful.
For this to be effective, the monstrous character can't just be plain - they have to be wildly, fascinatingly unacceptable - prodigiously, fantastically, sublimely ugly. If you're anxious about the character not being hot you'll never pull it off because the entire fantasy is about not being afraid.