Set a custom error per site collection

Dear all,

We are currently struggling with a problematic that I want to share with you.

What I would like is to set a custom error page (404 or 500) per site collection and not one generic at the web application level.

I think this is not possible to have it out of the box, so what about a custom development or a handler? Would that be possible with not a huge effort?

Thanks a lot for your help