It sounds like a good idea at first, but if people decide to make missiles to launch into Gaza, they will probably be arrested before they're even able to do anything. If they do get to launching any missiles, they will be found and arrested immediately. Even if they were able to evade the police and military, the people making the missiles wouldn't be experts, they would probably need to test the weapons first to make sure that they wouldn't cause harm to Sderot residents. The Gazan arabs may not be intelligent, but the Kassam makers have been building and "perfecting" how to make the projectiles for years. Anything that a Sderot resident is likely to whip up quickly will be wildly inaccurate and could potentially pose more harm to fellow residents than anyone in Gaza.
Better to protest like Israelis have been doing recently and ratchet up the pressure on the government to take action, as futile as that sounds.