Yes, Pete I've looked at a few after market jobs and you are right, the standard unit is pretty shallow. I don't have a problem with the unit, but the method the rim attaches to the bowl lacks accuracy.
The retaining screw is about 25mm from memory, and once this thin screw passes through the rim, you have to locate the thread which is about 10 mm behind the rim, so the rim has to be in the exact position and the screw a true 90 degrees to make a successful union. In daylight it took me a few attempts, but trying to do it in roadside conditions may be challenging. Let's hope the bulb doesn't blow!