Need advice on how to inset something into wood.

I did this using chisels, but I really think there’s got to be a better way to do this. I’m trying to hollow out the footprint for the viaduct leg to stand on. Is there a dremel tool or a bit for a drill press that would accomplish this? I know that a router and router table would work, but I don’t have one of those.

I think a router would do this in short order.