How to black out a window

[Wellinton]

Any tips to black out three windows on a metal door. Venue is donated for three weekends, it's not ours.
Black cardboard and duct tape didn't hold well and super adhesive tape left a mess of residue that we spent a long time cleaning. My husband wants to ask owner if we may paint the windows black, but I was hoping you might have another idea.

[sean]

cardboard with velcro...maybe a second window, painted black, attached with velcro

[screamforadream]

I've had GREAT success with window paint. You know, those bulky marker type things that people write "Just married" or "Prom 2012 Seniors!!" On their cars and whatnot? It went on and covered everything well and came off without any major residue, just use a straight razor and scrap it off and it'll be like it was never there.

[Laughing Jack]

Aluminum foil will not only black it out but it turns it into a mirror...

[Allen H]

I second the aluminum foil, I hold mine on with clear packing tape.

[Wellinton]

Excellent tips. Thank you everyone.