JAWUG

{joey}
I used instructions from Extremetech to build a few cantennae.

There are a few important aspects to a cantenna.
1) The can dimensions, ie. length and diameter
2) The position, shape and size of the feed element

There are different opinions on both aspects, but the general rule of thumb for can dimentiosn, seems to be that the ideal length of the can should be around 1 wavelength (approx 12cm) and the diameter around 10cm.
The element can be as simple as a 3cm piece of bicycle spoke (exact length can be calculated depending on channel, freq/4) or a wedge element, or conical. The latter two slightly improve the signal on different channels.
Conical feed element:

Cantenna with bicycle spoke element:

I also simulated the gain pattern of a can I used (Massbuilder) using 4NEC2 as discribed in extremetechs guide, see the pic below.