Is there a technique for making larger batches of roux? Following along the approach of freezing roux, I would like to make some in bulk. I am also trying to do more batch cooking for freezing and making 8 quarts of mac and cheese ends up taking a lot of roux for the starter.

As such, is there a technique for creating quite a few cups of roux (ie. 6 cups) at a time?

Currently the most I have created per batch is about 4 tablespoons butter to 4 tablespoons flour (~1/2 cup), mixing with a fork (pretty tiring as I have no whisk). Obviously a whisk could just be necessary, but is there a thresh-hold where moving to an electric or mechanical tool might be more efficient?
There is no problem at all doing this. At a restaurant where I worked, we would make up a couple pounds of butter worth of roux at a time. The main thing you want to do is use a pan with a lot of surface area, so it cooks evenly. A whisk will work fine. You don't have to stir constantly, just frequently.