Depending on the brand of trap, it may need to have a section of 1/2" hardware mesh added to the "floor" where the bobs hit the floor. The end is bent up about an inch, and you cut the last wire edge off, leaving the almost 1/2" single wires sticking up. The bobs will drop in between these wires, preventing the pigeons from pushing them sideways. They sometimes have enough play to move sideways, which provides just enough room for the pigeons to squeeze back out.
Here's a picture of a Tomahawk pigeon trap. You can click on the pic and zoom in to see the wires on this model, right where the bobs hit the floor. You can ad this panel using cage clips (rabbit cage clips with cage clip pliers), all available at any farm supply store.
http://www.hayneedle.com/product/tomahaw...p;tid=TMHK101-1I make my pigeon traps using the bobs from Foy's Pigeon Supply. Mine are the older wooden-framed style, but they now offer an aluminum design which looks like it would be great for building some quality pigeon traps.
Here's a pic of one of the traps I built:
This is the link to Foy's:
http://www.foyspigeonsupplies.com/traps-and-bobs/2139-american-aluminum-trap-6x12Keep in mind, the product above is sold as a "trap", but is ONLY the bobs and frame. It's NOT a complete pigeon trap. Terminology is different between pigeon enthusiasts and wildlife control people!