...That's why I wrote the Last Stand Encounter Builder.
With this tool you can create and edit Invader stats and powers, and then print them in nicely formatted cards or save them to PDF.
The powers are designed to be the same size as American Mini cards (so, the same as the little cards in Warhammer Fantasy, X-Wing Miniatures, and many other Fantasy Flight games) and will fit in card sleeves of that size. I also like to put an actual card in between the front and back to give them a little heft, but YMMV.
The Invader printout is designed to be an easy to read list of invader capabilities to use at the table. ...