Buff bot turret.
The bottom lane tower isn't actually any weaker than the other two lanes'. It's just that all towers were weakened and bot's happens to be the most accessible to the enemy team. *Note that it isn't often the bottom tower is taken within the first 5 minutes. Fortification serves only to make sure top and mid don't go down from a single kill.
All they'd need to do to fix this is increase that one tower's defenses and damage. There shouldn't just be "risk" involved in a 4-man gank, with a chance someone might die if they're stunned while getting targeted by the tower. It should be a guaranteed trade of 1 kill for the tower and most likely the dragon.
Something like the third or fourth tower shot from the bottom lane outer turret being a guaranteed kill would suffice. Since at the point where a tank should be able to survive 4 normal tower shots, that turret should be long since destroyed.