The adapter and the switch housing are taking too much space. So my advise is to totally redesign them, which should make the unit shorter.
Besides, that switch is so vulnerable. I had 4 switches already dead in a month! (I discovered 2 days ago that another adapter I had, was creating a shortcut, each time i put it in)
The switch can easily be replaced by a good old fashioned 2 metallic parts that touches that make contact each time one presses on the button. More reliability, not to mention that that's already probably something like 5 centimeters less in length for the whole unit.
I'm tired of waiting, so now while waiting for new switches, this is what I do
But I'll make a more proper contact point as soon as i have enough time.
So my suggestions:
1-Keep it at 6V and forget the resistor once and for all. It has no real use.
2-Lose that switch and replace it with 2 metallic contact points.
3-A led option wouldn't hurt.
4-Make it a bit shorter and slicker, but not at the cost of improvements.
But the best Mod ever to exist, would still be the one that integrates an automatic voltage switching.
I love the 4.3 Volts (or something) that the Passthrough delivers. It's a very nice voltage.