interdigital cavity filter

I need to design a 450MHz,with 20MHZ 3db bandwidth BPF.

I want to use cavity or interdigital filter,please help on design and mechanical sizes of the structure.

Is there any easy software or calculator for design this filter?

This is going to be a filter with big size. Why do you want to design it with cavity filter? It can also be done by LC filters, which of cause gives more insertion loss.

To evaluate the size, you need to specify your rejection requirement and insertion loss requirement.