Creating a gap or grove in a shallow angled corner

 From:  paulrus
6851.1 
Hi guys,

I'm building a model that has a lot of very straight edges that meet in very shallow angles. Where they meet, I need to create a squared off gap, or groove between the panels.

I've tried using inset and I just get "calculation failed".

Is there any other way to create something like that? I have a ton of them to do.

Thanks,

Paul