Nah, about a foot of wire would be half the wavelength. Maybe a little more. I'm too tired (and lazy. Yes, lazy. Screw you, hippie, it's Saturday.) to do the calculation. I know that those old 466mHz cordless phones had a wavelength that was right at 2'.
Grounding it wouldn't hurt, either, but I see that it's battery operated, so that may be trickier than it sounds. You'd need to attach extra antenna length to the antenna inside the box (or, alternatively, uncoil any internal antenna and hang it in a window). With a wavelength that short, there won't be an antenna coil inside (as in ferrite coil. A bunch of wire wrapped around an iron bar), just a length of wire *somewhere*. It'll be a length of at least a foot that seems to go off to nowhere. It might even be printed on a circuit board. If you open it up and take a decent photo of it, it'll be easy to identify. Just adding length to an antenna will help, but it's always best to add it in relation to the wavelength to get good results.
Just so you know, modification will completely void the warranty.
BTW, wavelength is calculated by dividing the speed of light by the frequency. So if the speed of light is roughly 30,000,000,000 cm/sec and the frequency is 434 million cycles per second (30 billion divided by 434 million), we get a wavelength of about 69cm, which is... uh, about 27".
Dammit, I did the calculation anyway.