Directional antennas typically tradeoff size for directionality. In addition, most good antenna's sizes are scaled as a function of frequency. In other words, the lower the frequency the larger the antenna.
There are several ways to get directionality at 433 MHz. One way is to make a Yagi-Uda beam antenna (usually just called a Yagi) which is a common type. Here is a link to manufacturer of several types.
**broken link removed**
In this chart you will find some Yagi models and you will see that their directionality gets better as you add more elements. For example, a 3 element yagi provides about 6.5 dB of directionality. This is still a fairly broad pattern. Even though its performance is modest, its size is still 16 x 14 x 5 inches. For better performance in a yagi, you have to add elements and you can only take it so far. For example, they offer an antenna that gives 15 dB of gain, but to get this, they put four antennas together in an array with a total size of 57 x 47 x 39 inches.
Another type worth considering is the corner reflector. They can build one that gives you 12 dB gain with a size of 30 x 23 x 50 inches. Another type is a semi-parabolic reflector which gives 15 dB of gain but its 46 x 25 x 88 inches. Such an antenna has a very narrow beam, about 18 degrees wide.
These commercial designs will give you an idea of what is possible. Since your range is only 8 meters, which is very little by the way, it will be hard to justify such a large antenna. We usually don't try to get directionality when sending RF such short distances. Why do you want directionality?
Making antennas is something of a precise art. Without the instruments necessary to fine tune a design, you pretty much have to stick to either simple conventional antennas using cookbook formulas, or specific recipes for more complex antennas using dimensions published in a magazine article or an online article. Often it is possible to scale a design for another frequency by multiplying all dimensions by the inverse of the frequency (lower frequency means larger dimensions).
Unfortunately, I do not have a specific site to point to that might offer some hope for a simple reflector box design, and certainely not something that is really small. Such a reflector box will likely work if it is, say, 60 to 100 cm on a side (thats about 24 to 39 inches roughly). It would be easiest to make it a lot like the so-called corner reflector, where you really only need two flat surfaces behind a dipole, the surfaces being joined along one edge at right angles.