I'm using both designs on my current 2 bbl kettle. 2 through-wall triclamp elements, and 2 heatsticks, all 6000 watt. You should also consider the tri-clamp bulkhead method. This would allow you to easily and completely remove the element and electrical box or housing and clean more freely. I used Kal's method of mounting the element in an outdoor box (I used a single gang box), and just used triclamp to connect the box and element to the kettle. You would need to weld or solder a 1.5" triclamp ferrule to your kettle.
Forget the option for permanent installation. You've already found out its drawbacks right?
I love my sticks too, but they are bulky. If I only needed one element, I'd say the best solution is the triclamp/gangbox.
Another argument for heatsticks is the ability to remove them and whirlpool.