I tried that. Wasn't durable. Kept losing connection. A single 9v battery hardly puts a dent into the starter but two good quality in series makes them go up in flames. The 9v batteries lasts many launches and cost about $1 each if you buy enough. Admittedly, I need to verify the voltage after last weekend's 14 launches.9V batteries are good, but an expensive way to solve the problem. They don't last very many launches considering their price because of that high internal resistance, and the fact that they're made up of a half-dozen tiny little batteries.
Perhaps a better solution might be: