I purchased an OnGuard lock yesterday from Sunrise Cyclery, who were fantastic do deal with. The lock, however, seems to have a bit of an identity crisis -- it's labeled as both a Pitbull and a Bulldog. I plan to take off the plastic guard and create what Sunrise advertises as a "Bulldog Mini Thin Crossbar," the lock that many have found for cheap through Amazon.
I have the lock in that picture, and I'm not sure you can take off the plastic. Maybe the middle silver part, but that's probably less than 1/8" thick and works as a keyhole cover, which is worth having around in sketchy weather.
I was able to take it off (per Onguard Pitbull lock fun), however mine is bare metal underneath. I have covered it with electrical tape for the time being, but I'll likely create a more permanent solution out of PVC. Now that I know, it would have been much easier to get the "thin crossbar" version available on Amazon.