Check out this project on RYOBI Nation - Needed a baby gate for the top of our basement stairs. I took out the existing basement door, and placed a 29" by 36" wooden baby gate. Made from 1x4 common boards, sanded it down, stained it Golden Oak, screwed it together with 1.25" wood screws, used a gate latch also.
Need to install a baby gate, but don't want to drill into the wooden stair banisters??? Cut a couple of 2x4's to the height of the banister, paint,and secure with heavy zip ties. Then just add the baby gate hardware to the new piece of wood........D.