Our team, while designing our bridge manipulator, wanted it to be as simple and intuitive to use as possible. We knew that an arm could have multiple uses, but would also need a driver to judge distances to work properly. Instead, we built a wedge that allows the driver to drive straight into the bridge and lower it.
Here's a picture of the wedge in its down position:
It's not perfect. The wedge had to be fit into a
very small section of our robot, and because of that it unfolds a little awkwardly sometimes in autonomous (but our human operation doesn't have a problem with it). We also ran into an issue where even though the wedge was supposed to be supported by our bumpers, it still rode up when hitting the bridge (actually pushing back into our main breaker in a practice match and shutting us off).
The solution we came up with to transfer maximum force and reduce the backdriving was a simple bar of angle iron stretching across the wedge near the bottom part of it. This pushes straight into the bumpers and keeps it a tight fit as the robot lowers the bridge.
Altogether it probably weighs 5-10 pounds and uses one window motor. I don't have the exact weight now since we added parts to the subsystem in the competition. But it allowed us to lower the bridge perfectly all weekend.