We had a similar issue and ended up not getting a safety gate at all. First we placed a sofa there, which worked for a few days before he figured out how to climb over it. We tried a few other things too, but nothing worked. Our stairs are also wide and none of the safety gates would fit. We ended up padding the sides and corners with a lot of cushions and just kept an eye on him.
We got child proof door handles for all of the doors (especially the bathrooms) so that he cannot open the doors and possibly lock himself in.