Well shoot. I tried lots of schemes to pull the edges outwards, but nothing really cleaned up the vertical waves.
Observation #1: these untensioned screens aren't too bad with single lens projectors, but are awful with three lens projectors due to color shift. With my single lens ECP Graphics, it wasn't so bad. But with a Marquee 8501LC, the color shift in the waves was so noticeable that black and white films became unwatchable. On one side of the wave it would be bluish, on the other side reddish.
Observation #2: I blew it off and ordered a Vutec Vu-easy screen. If a cheapskate like me buys another screen it must be for a reason.
Feline videophiles Condoleezza and Dukie