I know people have posted here about making weirs similar to the one you have out of metal. I do think it would be more than just the actual outflow - perhaps something with sides and a back that could be attached to your bog?
You can definitely buy these, but I think you're right - most are designed to be plumbed into a waterfall. What if you take your idea to a metal manufacturing place? We needed some heavy duty brackets to hold a large stone island in our kitchen and we were able to work directly with a metal worker who built them to our specs. Just an idea!