Update 1/24/17: Now supports pinched and formed output. See below for an example that makes a nice 40mm fan shroud.

This allows you to cover a hole with a specific diameter (with optional grating) and then attach a hose of a different diameter. For example, if you're creating an exhaust vent for a 40mm fan to push air out through a 1.25-inch tube, then you can render this file with a 40mm input diameter and a 1.25-inch output diameter. The angle and thickness of the transition portion is configurable to ensure good prints.

There are 15 parameters, making it pretty flexible. The 60 degree transition works very well on my printer and also looks pretty good.

I've added a few scad files for mounting 40mm or 80mm fans to 1.25" flexible hose. The 80mm version has not been tested, but I'm using the 40mm models to exhaust ABS fumes for one of my printers.

The green one in the photo was printed a tiny bit small so I added some tape for a snug fit.

If you're building an enclosure and need to plug holes and still allow cables to get through, try this thing: http://www.thingiverse.com/thing:38819

I use this as part of my parametric printer enclosure, which is available here: http://www.thingiverse.com/thing:39787

Use these settings for a nice 40 mm fan shroud:
roundRadius = 3;
thickness = 0.5;
flangeThickness = 1;
holeDistance = 32;
holeDiameter = 3.5;

boltHeadDiameter  = 6;angle             = 68;inputTubeLength   = 1;inputDiameter     = 37;outputLength      = 12;outputDiameter    = 14;meshLines         = 0;meshWidth         = 0;meshHeight        = 0;optimizeThickness = true;pinchAngle1       = 60; pinchAngle2       = 70;tipCutRadius      = 12; tipCutInset       = 1.5;