A good site to get some insights would be Deadwoodjedi. There you can see proposals for speeds. 

There are also tips how to set up such unkillable teams. I think HP is also one of the signifikant factors, not only def

I don't recognize those speeds but I'll assume they are right. But 1838 def on your slow boy is too high - to consistently be targeted for the stun, you really have to deliberately push their defense as low as possible. Something more like 1200 or so. 

Also, FB doesn't seem like a great choice for slow boy, because of how she only puts poisons while her poison sensitivity is up. Unless you can somehow extend her poison sensitivity with Vizier or so, having her so slow will mean she can't put much poison or do much damage... 

I think L9753 is right in that you need the defense of the slow boy to be much lower.  I also think you need the HP to be higher, as they need to have a higher percentage HP loss than the other team members.  Most of the guides I've read say the slow boy should be 125% of PK health, which would be closer to 70k if PK is 59K.

Essentially the goal is to make that particular champion as appealing as possible

On that point - they ALL have very high HP, for an unkillable. Pushing FB's HP even higher seems stupid, but PK and ME have a much higher HP than necessary, as well. 

In fact, the unkillable I built specifically says, though I don't think it's really a hard limit, that you should keep all of your champs' HP below 50k. The 125 percent thing isn't really an issue - my PK's HP is only a hair below my slow boy's HP, it still works. 

Rose can u pls gime your numbers on ME, PK and slow boy.

Regarding Frozen as slow boy- it was just a temporary solution until i grow another champ for that position.

Hey Warsong,

I wouldnt recommend FB as slow boi, she contributes to so much damage as her a1 has potential to land more poison debuffs. 

Do you have discord, I could show you my budget unkillable team. and we can go through your options