Like lolzy said, just a on/off switch. Maybe you could use PTCT and just one wire so after it's sparked once, it'll go to the off switch, instead of using two wires. Am I making sense..? Hold on, I'll do what I'm talking about and upload it.

Right. So I kinda did it, but it's way too big (4 logic gates right now). My OCD compels me to make wires the width of four pixels..so I guess you could cut down there too. Finally, three of my logic gates use the old system, and not semiconductors. If you used semiconductors for those, you could make it smaller.

The thing is, it only works once, which means after you switch the e-wall on and off, you can't do turn it back on again. I can probably fix this, but it'll be later or even tomorrow.

I'm sure there's a much better way to do this but I probably can't.
Anyway, here's the link.
http://powder.hardwired.org.uk/View.html?ID=5417

And thanks for the challenge!