Fretless Simulator?

If you include double bass, I've been playing fretless basses for about 60 years, and the sound each bassist envisions as "the fretless sound" seems to vary quite a bit.

I played a fretless five-string exclusively in one band for about a decade, and other bassists seemed to he the only folks who commented. To my band mates, it was just a bass.

Bass The World recently reviewed a fretless pedal on their YouTube channel. Perhaps it includes some of the sounds you are after.
 
To add to what JazzDog was saying: YEARS ago in my 1st band the guitarist bought a boss ac-2 that didn't mesh with him but he was DETERMINED to find a use for it. He snuck it into the fx loop of my amp and did made a decent chorus/verb for our ballady stuff. A few drunk guys along our run, since I changed basses for these song came up to me and said how "glorious that fretless sounded" it wasn't a fretless at all..
 
Just get a real fretless and put in some work. It takes time, but you can do everything a fretted bass can do and a lot more.

If you really just don't want to put in the effort, get your frets replaced with the smallest "mandolin" frets you can find and work on your glissando and vibrato techniques to try to mimic the fretless feel. Put some chorus on it and you'll have a cool sound, at least.

Either, there's no free lunch (that tastes any good, at least).
 
I switched to fretless in 2009. Found an ESP fretless 5 with a 2Tek bridge at a very affordable cost. What i discovered very quickly was that if you’re playing a fretted bass and your intonation is such that you’re playing really close to the frets, theres a short learning curve. I think it possible that folks having difficulty with switching to fretless are used to playing in the middle, say halfway between the frets. The result is a flat mess. I had no trouble, and I wasn’t even remotely adept at playing when i switched. Its hard to get worse when you weren’t any good to begin with.