This page contains descriptions of the variations in port designs used in speaker enclosures.
The most common type of port is a round tube. These are typically made of black plastic and look like a section of thin-walled pipe with a lip on one end.
The second most frequently used type of vent is a square port. The advantage of this design is that you are not limited by availability of only certain diameter tubes. You can also build the port out of the same material you are using to fabricate the enclosure.
The last type of vent is called a slot port. It is created using one wall of the enclosure as a wall of the vent. This can be very useful for bandpass enclosures which can be very hard to tune to a low enough frequency due to the small size of the front (vented) chamber. Friction created by the air flow in the enclosure traveling along the enclosure wall makes this type of port effectively longer than it physically is - which tunes the enclosure to a lower frequency so a slot port will be shorter for the equivalent vent area when compared to a simpler square vent.
Keep the port(s) as far away from the enclosure sides and back as possible except for the slot type port.
Make sure the inside end of a tube or square port is at least the equivalent of one vent diameter away from the back of the box or any bracing materials.
Keep the vent free from anything that may affect air flow through it.
The port should also be placed as far away from the speaker cone as practical.