To convert from a tube port to a slot port (round to rectangular) you need to 1) maintain the same area of the port and 2) same port volume. If you do these thing, the port will work fine (although slot ports tend to exhibit more port noise at times, since people tend to put them at the edges of the box).

One benefit of a slot port is that it can be "folded" (although you MUST be sure to remove the port volume from the volume of your enclosure, when calculating your enclosure size). A folded port can have a much greater area and volume than a tube port; and this will lead to much lower box losses (lower port Q) and tighter, cleaner bass output (particularly around the tuning frequency).

WJ