Russell Bateman April 2016 last update:
...in December, 2015, living above our bedroom in the attic. We were unsuccessful.
We purchased a Havahart 1030, two-door, 24" × 7" × 7" trap (below) with peanut butter. The squirrel appreciated the peanut butter, but never sprang the trap. Tried it two-door and one-door. One thing is certain: we never had any trouble enticing the squirrel to eat the bait.
We tried peanut butter in a little low-profile cup anchored to the trip plate and once on parchment paper directly on the plate.
In April, 2016, I ordered a Havahart 1025, two-door, 17½" × 5" × 7" trap. This trap is much smaller than the other one I bought though it looks almost identical to it.
These traps cost about $32 and $23 respectively.
I saw a video advising setting out bait either below grade under the back of trap outside (not our case) or in a PVC pipe almost as wide as the trap itself hanging in the inside just above and slightly beyond the trip plate.
This is what I'm going to try next except that I worry the animal will attempt to reach the bait inside the PVC pipe from the outside through holes. I might cover the ends of the pipe with caps and cut instead a slot in the pipe across lengthwise to entice the squirrel to reach it from inside the trap.
We've tried a smaller trap. We've also learned how to set the trap to spring a little more easily. Here the first illustration is the incorrect way. The second is the right way to do it (click to enlarge):
Here I built a feeder to use in place of the trigger platform that would be very much harder for the squirrel to get peanut butter from without stepping on the platform.
And here is how it's installed. In the second illustration, the trap is upside down. The ½" PVC tube slides through the hole in the Havahart 1030. I saw this trick elsewhere on the web. Sadly, our squirrel is way more intelligent. He sprang the trap the last two times while getting all the peanut butter and got away. My present theory is that his body is long enough that he keeps the sprung trap door open while eating the bait, then is able to back out of the trap.
I'm installing a security camera in the attic aimed at the trap for the next attempt. I've got to see the World's Most Intelligent and Clever Squirrel for myself.