433mhz micro directional antenna design
You are going to find it difficult to come up with any truly directional antenna that small for 433 MHz.  That is basically the 70cm band, so a full wavelength is going to be 70cm.  Even a shortened loaded yagi type antenna won't fit in your box.

About the smallest, efficient, directional antenna I can think of would be the Moxon design, but even that is too big for your application, by a great amount.

Your best bet would probably be to just use a standard "Rubber Ducky" antenna mounted to a BNC fed through the box.