It’s raining a lot here in my town and I put a big 150-liter bucket under House gutter (spot that drains rainwater from the roof) and this bucket fills and overflows I can soak the utensils in the rainwater bucket that’s overflowing do?
There are numerous conditions to making a kosher mikva, and it usually needs the guidance of an expert in these halachos to give guidance when making a mikva. Regarding your question the container that you have is not a valid mikva, and here are some of the reasons why. The first reason is that the mikva must be attached to the ground and not inside a vessel. Another condition is that the mikva has to contain 40 seah of water inside it, which is big enough for a person to immerse inside it. Meaning that it has to be approximately half a meter by half a meter by one and a half meters tall, which comes out, which is approximately 330 liters (and according to some authorities 60 cm. by 60. cm by 180 cm, which would be a lot more than that). Therefore, even according to the smallest opinion the container doesn’t quite have enough water in it.
Therefore you would be well advised to bring the utensils that need tevila to a mikva that is certified by competent authorities that it is indeed a kosher mikva.
Shiur Torah pg. 257, Shiurin Shel Torah.