The thought of Gizmo balancing on the outside of the railing absolutely makes my knees weak. We have a split level foyer & Huey likes to look through the rails but it would be a 15 foot drop if he fell & we had to block off that area until we trained him to not do that.
A cheaper although not as asthetically pleasing
temporary solution could be some plastic netting between the stair rails.
Garden Netting from Industrial Netting
Is there a chance that he will get big enough to not fit between the rails?
The cats will have to find an alternate route, though.