Instead of focusing on making it temporary, why not focus on making it work better... every bottle dynamo I have used seemed to cause massive drag (at least compared to no dynamo or a properly working hub dynamo). If you had an old hub dynamo (old sturmey archer) you could focus on rewiring it to have a more stable power supply (I have heard these used to blow up lightbulbs when you went down a big hill), and making charging jacks to plug your devices into?