It is difficult to pour from a full gallon can without spilling some.
Here's a remix. The spout swivels. However, it's somewhat of a work in progress. There may be a better way to turn the spout.
http://www.thingiverse.com/thing:1223758

Spout for Gallon Cans

It can be difficult to pour without spilling from a full gallon can.

I bought a new gallon can of WD-40 and of course it's full to the brim, and in trying to fill my spray bottle's tiny hole (with a funnel) I still spilled quite a bit as the liquid glug-glugs out.

So, I searched thingiverse but could not find this spout. Here's my version. If someone already posted this spout then credit to them.

It fits the one gallon cans of acetone, paint thinner (mineral spirits), alcohol, lacquer thinner, WD-40 etc., found in paint departments in the big box hardware stores. At least it fits all of my cans. I think they use the same cap-thread on all the metal, rectangular gallon cans (in the USA) but who knows.

The critical surface is where the can's lip meets the spout. That should be flat and liquid tight.
But Afinia software insists on putting extra layers of "surface support" under such horizontal surfaces. Sometimes it's not easy to get in there and pry that support off the bottom of a cavity, and it often leaves a rough surface.

Printing this spout upside down eliminates that, but requires an extra 4 grams of exterior support material.

using afinia defaults:
Right side up: 38 min, 14.5 g material. (5g support)
Upside down: 49 min, 18.5 g material. (9g support)

Definitely print in nylon or some other chemically resistant material if you plan on using this spout on anything like acetone or lacquer thinner.