Even slot ports should be flared.

The normal method of flaring a slot port is to use a rollover bit in the router to run all the way around the port opening.

If you are moving a lot of air, you may find that this is not sufficient.

One solution is to take a length of PVC pipe and split it lengthwise into quarters.

These segments can then be let into the enclosure to form a rectangular flare.

The corners would be mitred at 45 degrees.

Even less work is involved if you use half segments, as shown in this photo. Not quite as neat, but effective just the same.

When calculating usable velocity, you need to make allowance for the corners being less streamlined than the rest of the flare.

Modelling the flare radius as 25% smaller should do the trick. Note that the flare uses the outside diameter of the pipe as the flare

This table shows what would be achievable starting with common pipe sizes

Smoothly flared corners, could be made using a heated sheet of PVC pressed between male and female molds.

The edges would need to be cleaned up with a router.

This would give the same shape flare that would be achieved with a router and an impossibly huge rollover bit.

It would improve the performance of the corners, although they would never be as good as a circular port.

Sketchup .skp file

If you would like to know how to do rolled edges in Google Sketchup, see my tutorial

I'm only guessing, but I would say that modelling the flare radius as 20% smaller should be reasonable

This would adjust our table as follows

Segments of pipe can also be used to smooth any turns inside a slot port

In this example, you would need two pipe sizes.

After being split and heated, PVC pipe is quite amenable to being wrapped over a suitable former to whatever radius you need.

If you are increasing the radius of a segment of pipe, remember to initialy cut the segment larger than 90 degress

You could go even further with this idea