Is it 45 cm ? Would it fit a unger rectangular bucket?

Hi Jessy! This will, but not perfectly, and we don't typically recommend this. Instead we would suggest the Unger Bucket Sieve.

Hello! I like this item. Do you have an instruction manual on how to apply this to a bucket ? Thank you!

Here is a super easy-to-follow instructional video! https://youtu.be/jHbudaBEwZ0