Some of us call that a router sled. I built one that was about 4' across and 8' long once so that I could flatten the top of the bench I built out of laminated 2 x lumber. If you get wide enough, like I did, then you need to reinforce the cross piece of the sled with sides to keep it rigid enough to prevent deflection, basically a U shaped box.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.