Daylight Long Exposure stacking

I'm hoping that someone will be able to help direct me to a suitable alternative to PS (Not paying for that on top of CO pro... but willing to pay a little more for the right bit of kit..) as CO doesn't offer image stacking..

As well as using my Canon 5D, I sometime use a drone for photography.. and obviously by its nature, long exposures with a drone are just a no go, as stable as they may seem to some.

So I'm looking for software that will allow me to stack/merge/mean say 5-10 images to achieve a long exposure effect in water, clouds, lights etc...

The closest I have found is a mac bit of kit, Which you can see here: https://sites.google.com/site/longexposurestacker/home  but they do not offer a windows version...

Any advice or direction pointing would be much appreciated..

Paul